Add bit field support

[PathogenDavid]

The libclang Pathogen extensions provide information about bit fields, but we don't really use it beyond marking the fields are unusable.

I think the best approach here is to make an additional TranslatedBitField type to represent them rather than trying to wedge them into TranslatedNormalField. That way the relative edge-case bit field stuff can live separate from the extremely common case of normal fields.

C# does not support bit fields, so we'd need to implement them manually using properties. This should be fine since the only thing this really does is prevent taking a reference to the bit field, which isn't something you can do regardless.

[PathogenDavid]

Bit field support is complete. It hasn't been rigorously tested yet, but it's assumed that'll come with #32 or something similar.

BitFields.h contains a small suite of bit fields covering typical and edge case scenarios.

#71 was opened to cover an extreme edge case that was not covered by the initial implementation. (The output won't be totally broken in this scenario, but the values you get from a bitfield getter might not be sign-extended when they would've been in C.)