[String] Scalar-alignment bug fixes.

Fixes a general category (pun intended) of scalar-alignment bugs
surrounding exchanging non-scalar-aligned indices between views and
for slicing.

SE-0180 unifies the Index type of String and all its views and allows
non-scalar-aligned indices to be used across views. In order to
guarantee behavior, we often have to check and perform scalar
alignment. To speed up these checks, we allocate a bit denoting
known-to-be-aligned, so that the alignment check can skip the
load. The below shows what views need to check for alignment before
they can operate, and whether the indices they produce are aligned.

┌───────────────╥────────────────────┬──────────────────────────┐
│ View          ║ Requires Alignment │ Produces Aligned Indices │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Native UTF8   ║ no                 │ no                       │
├───────────────╫────────────────────┼──────────────────────────┤
│ Native UTF16  ║ yes                │ no                       │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Foreign UTF8  ║ yes                │ no                       │
├───────────────╫────────────────────┼──────────────────────────┤
│ Foreign UTF16 ║ no                 │ no                       │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ UnicodeScalar ║ yes                │ yes                      │
├───────────────╫────────────────────┼──────────────────────────┤
│ Character     ║ yes                │ yes                      │
└───────────────╨────────────────────┴──────────────────────────┘

The "requires alignment" applies to any operation taking a
String.Index that's not defined entirely in terms of other operations
taking a String.Index. These include:

* index(after:)
* index(before:)
* subscript
* distance(from:to:) (since `to` is compared against directly)
* UTF16View._nativeGetOffset(for:)

test/stdlib/StringTraps.swift

@@ -168,17 +168,5 @@ StringTraps.test("UTF8ViewIndex/offsetCrash")
   _ = s8.utf8[i]
 }
 
-StringTraps.test("String.Index.utf16Offset(in:)/subscalarUTF8")
-  .skip(.custom(
-    { _isFastAssertConfiguration() },
-    reason: "this trap is not guaranteed to happen in -Ounchecked"))
-  .code {
-  let s = "😇"
-  let u8 = s.utf8
-  let i = u8.index(after: u8.startIndex)
-  expectCrashLater()
-  _ = i.utf16Offset(in: s)
-}
-
 runAllTests()