Replace parser functions with methods

[TheDan64]

and add wrappers for public, no_mangle functions

[rinon]

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.

[TheDan64]

It's still early, but going well so far. Currently trying to figure out how to get the FREE!, MALLOC!, etc macros to play nice with self. Once I get to the freeing code then maybe it'll get a little hairy. If your code is ready to merge we should do that soon before I get too far into it

[rinon]

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 of the parser then passing the memory allocator struct as an expr is better.

[TheDan64]

This should be mostly done. There are still some remaining functions, mostly small but public-facing ones.