Add PureXML.Parsing.ByteEncoder, the inverse of ByteDecoder, and a
Serializer.serialize(_:encoding:) / PureXML.serialize(_:encoding:) that
emits bytes in a chosen encoding (the libxml2 save-with-encoding model).
Covers the Unicode transformation formats and the single-byte legacy
families, built by inverting the existing vendored to-Unicode tables, so
no new data is introduced. The declaration carries the canonical encoding
name, a UTF-16/32 stream is preceded by a byte-order mark, and any
unrepresentable scalar becomes a decimal numeric character reference, so
output round-trips. CJK output encodings remain (tracked on #106).
Part of #106, #105
Mihaela Mihaljevic
e62995a7a8