[Stdlib] Eliminate the _UnicodeEncoding_ protocol.

This protocol was only used to fake recursive protocol conformances;
collapse it into _UnicodeEncoding.

Author: DougGregor

stdlib/public/core/UTFEncoding.swift

@@ -17,7 +17,7 @@
 
 
 public protocol _UTFParser {
-  associatedtype Encoding : _UnicodeEncoding_
+  associatedtype Encoding : _UnicodeEncoding
 
   func _parseMultipleCodeUnits() -> (isValid: Bool, bitCount: UInt8)
   func _bufferedScalar(bitCount: UInt8) -> Encoding.EncodedScalar

stdlib/public/core/UnicodeEncoding.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-public protocol _UnicodeEncoding_ {
+public protocol _UnicodeEncoding {
   /// The basic unit of encoding
   associatedtype CodeUnit : UnsignedInteger, FixedWidthInteger
 
@@ -44,12 +44,12 @@ public protocol _UnicodeEncoding_ {
   /// A type that can be used to parse `CodeUnits` into
   /// `EncodedScalar`s.
   associatedtype ForwardParser : Unicode.Parser
-  // where ForwardParser.Encoding == Self
+    where ForwardParser.Encoding == Self
 
   /// A type that can be used to parse a reversed sequence of
   /// `CodeUnits` into `EncodedScalar`s.
   associatedtype ReverseParser : Unicode.Parser
-  // where ReverseParser.Encoding == Self
+    where ReverseParser.Encoding == Self
 
   //===--------------------------------------------------------------------===//
   // FIXME: this requirement shouldn't be here and is mitigated by the default
@@ -60,15 +60,10 @@ public protocol _UnicodeEncoding_ {
   static func _isScalar(_ x: CodeUnit) -> Bool
 }
 
-extension _UnicodeEncoding_ {
+extension _UnicodeEncoding {
   // See note on declaration of requirement, above
   public static func _isScalar(_ x: CodeUnit) -> Bool { return false }
-}
-
-public protocol _UnicodeEncoding : _UnicodeEncoding_
-where ForwardParser.Encoding == Self, ReverseParser.Encoding == Self {}
 
-extension _UnicodeEncoding_ {
   public static func transcode<FromEncoding : Unicode.Encoding>(
     _ content: FromEncoding.EncodedScalar, from _: FromEncoding.Type
   ) -> EncodedScalar? {

stdlib/public/core/UnicodeParser.swift

@@ -43,7 +43,7 @@ extension Unicode {
 /// scalar values.
 public protocol _UnicodeParser {
   /// The encoding with which this parser is associated
-  associatedtype Encoding : _UnicodeEncoding_
+  associatedtype Encoding : _UnicodeEncoding
 
   /// Constructs an instance that can be used to begin parsing `CodeUnit`s at
   /// any Unicode scalar boundary.