Stephen Crane

Handler fields were replaced in #9.

`m_initEncoding`, `m_unknownEncodingMem`, and `m_internalEncoding` were made safe in #19.

How’s this going? I’ve got changes on https://github.com/immunant/libexpat-rs/tree/encoding_references that I’m sure will conflict, so wanted to coordinate.

Eventually we want to remove those macros, but in the mean time, just pass self as an ident in place of parser. If you need to use the macro outside...