`YGStyle`: Make getters/setters template args of `BitfieldRef`
`YGStyle`: Make getters/setters template args of `BitfieldRef`

Summary:
@public

Makes bitfield getters/setters part of the bitfield ref template.
Since we introduced the tracking bit as template parameter in D14933022, every bitfield ref is an individual class anyway, and having function pointers doesn’t potentially lead to less code generation anyway.

Furthermore, this change can (in the absence of tracking bits) avoid less specialized templates dealing with refs, and to dynamic dispatch as a consequence.

Reviewed By: SidharthGuglani

Differential Revision: D15085495

fbshipit-source-id: 0dd70fa05e9d43a29e38a619cddb642c9ca3f7ab

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