Pitch(Foundation): String Encoding Names

[benrimmington]

@YOCKOW

If we want strict correspondence between names and encoding/decoding implementations, we could get inspired by @Karl's suggestion …

ByteStreamDecoder might be a better alternative than Unicode.Encoding because:

• endianness and byte-order marks could be supported,
• conformance wouldn't require both ForwardParser and ReverseParser,
• name lookup could return an instance rather than a metatype.