[codegen] Add wrappers on enums for shared methods

[cmyr]

This is for tables that have multiple formats, where all the formats have an identically named method; we will now generate a wrapper method on the enum itself that forwards to the inner methods.

In this patch this is only implemented for raw fields, and we do not generate the typed offset getters. This is quite a bit more complicated, and we can look into that as followup.

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This ended up producing less code than I had expected, and most of that code is just exposing the format field. The two main 'real' cases are the bbox on glyphs and the coords on anchor tables, both of which we had previously hand-written implementations for.

The main thing that we are missing here (pending support for offset getters) is resolving coverage tables on multi-format layout subtables.

progress on #687

[dfrg]

I’m tentatively approving because the implementation is good and this was a requested enhancement. I do have a slight concern that this might lead to suboptimal code… for example, extracting a glyph bbox generating four match expressions instead of one. I’d be curious to see the optimized llvm/asm to determine if these branches will be combined.

[cmyr]

Interesting point. This does match the existing implementation, which was just using a macro_rules macro to generate these match statements, but if this ends up being a bottleneck we can always hand-write the more efficient version.

[dfrg]

Sounds good to me. 🚢