stdlib: Add unavailable declarations for APIs renamed in SE-0118

Sequence.sort(_:) => sorted(by:)
Sequence.sort() => sorted()
Sequence.elementsEqual(_:isEquivalent) => elementsEqual(_:by:)
Sequence.contains(_:) => contains(where:)
Sequence reduce(_:combine:) => reduce(_:_:)
UnicodeCodec.encode(_:output:) => encode(_:into:)
ManagedBuffer.create(_:initialValue:)
  => create(minimumCapacity:makingHeaderWith:)
ManagedBufferPointer.init(bufferClass:minimumCapacity:initialValue)
  => init(bufferClass:minimumCapacity:makingHeaderWith:)

stdlib/public/core/Unicode.swift

@@ -1140,6 +1140,16 @@ extension UnicodeCodec {
 @available(*, unavailable, renamed: "UnicodeCodec")
 public typealias UnicodeCodecType = UnicodeCodec
 
+extension UnicodeCodec {
+  @available(*, unavailable, renamed: "encode(_:into:)")
+  public static func encode(
+    _ input: UnicodeScalar,
+    output put: (CodeUnit) -> Void
+  ) {
+    Builtin.unreachable()
+  }
+}
+
 @available(*, unavailable, message: "use 'transcode(_:from:to:stoppingOnError:into:)'")
 public func transcode<Input, InputEncoding, OutputEncoding>(
   _ inputEncoding: InputEncoding.Type, _ outputEncoding: OutputEncoding.Type,