[stdlib] implement _copyContents for Slice

`Sequence._copyContents(initializing:)` is the function relied on by
`UnsafeMutableBufferPointer` for performant initialization from Collections.
Until now, `Slice<Base: Collection>` has not had its own implementation,
and therefore fell back to the default version implemented on `Sequence`.

This implementation adds an attempted fast path, using
`withContiguousStorageIfAvailable`. If that fails, the `Sequence`
algorithm is used, as before.

This resolves https://bugs.swift.org/browse/SR-14491

Author: glessard

stdlib/public/core/Slice.swift

@@ -229,6 +229,23 @@ extension Slice: Collection {
   }
 }
 
+extension Slice {
+  @inlinable
+  public __consuming func _copyContents(
+      initializing buffer: UnsafeMutableBufferPointer<Element>
+  ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
+    if let (_, copied) = self.withContiguousStorageIfAvailable({
+      $0._copyContents(initializing: buffer)
+    }) {
+      let distance = buffer.distance(from: buffer.startIndex, to: copied)
+      let i = index(startIndex, offsetBy: distance)
+      return (Iterator(_elements: self, _position: i), copied)
+    }
+
+    return _copySequenceContents(initializing: buffer)
+  }
+}
+
 extension Slice: BidirectionalCollection where Base: BidirectionalCollection {
   @inlinable // generic-performance
   public func index(before i: Index) -> Index {