[stdlib] Add Sequence.Element, change ExpressibleByArrayLiteral.Element to ArrayLiteralElement (#8990)

* Give Sequence a top-level Element, constrain Iterator to match

* Remove many instances of Iterator.

* Fixed various hard-coded tests

* XFAIL a few tests that need further investigation

* Change assoc type for arrayLiteralConvertible

* Mop up remaining "better expressed as a where clause" warnings

* Fix UnicodeDecoders prototype test

* Fix UIntBuffer

* Fix hard-coded Element identifier in CSDiag

* Fix up more tests

* Account for flatMap changes

stdlib/public/core/HashedCollections.swift.gyb

@@ -759,7 +759,7 @@ public struct Set<Element : Hashable> :
   ///
   /// - Parameter sequence: The elements to use as members of the new set.
   public init<Source : Sequence>(_ sequence: Source)
-    where Source.Iterator.Element == Element {
+    where Source.Element == Element {
     self.init(minimumCapacity: sequence.underestimatedCount)
     if let s = sequence as? Set<Element> {
       // If this sequence is actually a native `Set`, then we can quickly
@@ -827,7 +827,7 @@ public struct Set<Element : Hashable> :
   ///   otherwise, `false`.
   @_inlineable
   public func isSubset<S : Sequence>(of possibleSuperset: S) -> Bool
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     // FIXME(performance): isEmpty fast path, here and elsewhere.
     let other = Set(possibleSuperset)
     return isSubset(of: other)
@@ -855,7 +855,7 @@ public struct Set<Element : Hashable> :
   ///   `possibleStrictSuperset`; otherwise, `false`.
   @_inlineable
   public func isStrictSubset<S : Sequence>(of possibleStrictSuperset: S) -> Bool
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     // FIXME: code duplication.
     let other = Set(possibleStrictSuperset)
     return isStrictSubset(of: other)
@@ -878,7 +878,7 @@ public struct Set<Element : Hashable> :
   ///   otherwise, `false`.
   @_inlineable
   public func isSuperset<S : Sequence>(of possibleSubset: S) -> Bool
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     // FIXME(performance): Don't build a set; just ask if every element is in
     // `self`.
     let other = Set(possibleSubset)
@@ -904,7 +904,7 @@ public struct Set<Element : Hashable> :
   /// - Returns: `true` if the set is a strict superset of
   ///   `possibleStrictSubset`; otherwise, `false`.
   public func isStrictSuperset<S : Sequence>(of possibleStrictSubset: S) -> Bool
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     let other = Set(possibleStrictSubset)
     return other.isStrictSubset(of: self)
   }
@@ -924,7 +924,7 @@ public struct Set<Element : Hashable> :
   /// - Returns: `true` if the set has no elements in common with `other`;
   ///   otherwise, `false`.
   public func isDisjoint<S : Sequence>(with other: S) -> Bool
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     // FIXME(performance): Don't need to build a set.
     let otherSet = Set(other)
     return isDisjoint(with: otherSet)
@@ -955,7 +955,7 @@ public struct Set<Element : Hashable> :
   /// - Returns: A new set with the unique elements of this set and `other`.
   @_inlineable
   public func union<S : Sequence>(_ other: S) -> Set<Element>
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     var newSet = self
     newSet.formUnion(other)
     return newSet
@@ -976,7 +976,7 @@ public struct Set<Element : Hashable> :
   /// - Parameter other: A sequence of elements. `other` must be finite.
   @_inlineable
   public mutating func formUnion<S : Sequence>(_ other: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     for item in other {
       insert(item)
     }
@@ -998,13 +998,13 @@ public struct Set<Element : Hashable> :
   /// - Returns: A new set.
   @_inlineable
   public func subtracting<S : Sequence>(_ other: S) -> Set<Element>
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     return self._subtracting(other)
   }
 
   @_versioned
   internal func _subtracting<S : Sequence>(_ other: S) -> Set<Element>
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     var newSet = self
     newSet.subtract(other)
     return newSet
@@ -1024,12 +1024,12 @@ public struct Set<Element : Hashable> :
   ///
   /// - Parameter other: A sequence of elements. `other` must be finite.
   public mutating func subtract<S : Sequence>(_ other: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     _subtract(other)
   }
 
   internal mutating func _subtract<S : Sequence>(_ other: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     for item in other {
       remove(item)
     }
@@ -1053,7 +1053,7 @@ public struct Set<Element : Hashable> :
   /// - Returns: A new set.
   @_inlineable
   public func intersection<S : Sequence>(_ other: S) -> Set<Element>
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     let otherSet = Set(other)
     return intersection(otherSet)
   }
@@ -1073,7 +1073,7 @@ public struct Set<Element : Hashable> :
   /// - Parameter other: A sequence of elements. `other` must be finite.
   @_inlineable
   public mutating func formIntersection<S : Sequence>(_ other: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     // Because `intersect` needs to both modify and iterate over
     // the left-hand side, the index may become invalidated during
     // traversal so an intermediate set must be created.
@@ -1108,7 +1108,7 @@ public struct Set<Element : Hashable> :
   /// - Returns: A new set.
   @_inlineable
   public func symmetricDifference<S : Sequence>(_ other: S) -> Set<Element>
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     var newSet = self
     newSet.formSymmetricDifference(other)
     return newSet
@@ -1133,7 +1133,7 @@ public struct Set<Element : Hashable> :
   /// - Parameter other: A sequence of elements. `other` must be finite.
   @_inlineable
   public mutating func formSymmetricDifference<S : Sequence>(_ other: S)
-    where S.Iterator.Element == Element {
+    where S.Element == Element {
     let otherSet = Set(other)
     formSymmetricDifference(otherSet)
   }