stdlib: make map() dynamically dispatched

This change tries to recover the performance regression in map() that
was caused by moving map() to a protocol extension and degrading the
static type information (when mapping a collection, we only know that it
is a sequence).  Adding map() to the witness table allows us to provide
a specialized implementation for collections, and hopefully recover the
lost performance.

This is a speculative change, I don't have performance numbers.  I will
watch the performance buildbots and if this change does not help, I'll
revert.

Swift SVN r27607

stdlib/public/core/SequenceAlgorithms.swift.gyb

@@ -378,23 +378,6 @@ extension SequenceType {
   }
 }
 
-//===----------------------------------------------------------------------===//
-// map()
-//===----------------------------------------------------------------------===//
-
-extension SequenceType {
-  /// Return an `Array` containing the results of mapping `transform`
-  /// over `self`.
-  final public func _prext_map<T>(
-    @noescape transform: (${GElement}) -> T
-  ) -> [T] {
-    // Cast away @noescape.
-    typealias Transform = (${GElement}) -> T
-    let escapableTransform = unsafeBitCast(transform, Transform.self)
-    return Array<T>(lazy(self).map(escapableTransform))
-  }
-}
-
 //===----------------------------------------------------------------------===//
 // flatMap()
 //===----------------------------------------------------------------------===//