decode_c converted to ByteString

decode_c converted to ByteString

This speeds up a few things, notably CmdLine.Seek using Git.Filename
which uses decode_c and this avoids a conversion to String and back,
and probably the ByteString implementation of decode_c is also faster
for simple cases at least than the string version.

encode_c cannot be converted to ByteString (or if it did, it would have
to convert right back to String in order to handle unicode).

Sponsored-by: Brock Spratlen on Patreon