[stdlib] use a primary associated type

Author: glessard

stdlib/public/core/UnsafeBufferPointerSlice.swift

@@ -26,9 +26,7 @@ extension Slice where Base == UnsafeMutableRawBufferPointer {
   ///   be less than or equal to this buffer slice's `count`.
   @inlinable
   @_alwaysEmitIntoClient
-  public func copyBytes<C: Collection>(
-    from source: C
-  ) where C.Element == UInt8 {
+  public func copyBytes(from source: some Collection<UInt8>) {
     let buffer = Base(rebasing: self)
     buffer.copyBytes(from: source)
   }

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -534,9 +534,7 @@ extension Unsafe${Mutable}RawBufferPointer {
   /// - Parameter source: A collection of `UInt8` elements. `source.count` must
   ///   be less than or equal to this buffer's `count`.
   @inlinable
-  public func copyBytes<C: Collection>(
-    from source: C
-  ) where C.Element == UInt8 {
+  public func copyBytes(from source: some Collection<UInt8>) {
     guard let position = _position else {
       return
     }