convert encode_c to ByteString

convert encode_c to ByteString

This turns out to be possible after all, because the old one decomposed
a unicode Char to multiple Word8s and encoded those. It should be faster
in some places, particularly in Git.Filename.encodeAlways.

The old version encoded all unicode by default as well as ascii control
characters and also '"'. The new one only encodes ascii control
characters by default.

That old behavior was visible in Utility.Format.format, which did escape
'"' when used in eg git-annex find --format='${escaped_file}\n'
So made sure to keep that working the same. Although the man page only
says it will escape "unusual" characters, so it might be able to be
changed.

Git.Filename.encodeAlways also needs to escape '"' ; that was the
original reason that was escaped.

Types.Transferrer I judge is ok to not escape '"', because the escaped
value is sent in a line-based protocol, which is decoded at the other
end by decode_c. So old git-annex and new will be fine whether that is
escaped or not, the result will be the same.

Note that when asked to escape a double quote, it is escaped to \"
rather than to \042. That's the same behavior as git has. It's
perhaps somehow more of a special case than it needs to be.

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