[stdlib] Inline legacy UTF8.decode

Author: Dave Abrahams

stdlib/public/core/Unicode.swift

@@ -190,6 +190,7 @@ extension _Unicode.UTF8 : UnicodeCodec {
   /// - Returns: A `UnicodeDecodingResult` instance, representing the next
   ///   Unicode scalar, an indication of an error, or an indication that the
   ///   UTF sequence has been fully decoded.
+  @inline(__always)
   public mutating func decode<I : IteratorProtocol>(
     _ input: inout I
   ) -> UnicodeDecodingResult where I.Element == CodeUnit {