Implement codegen support for repr transparent.

[emilio]

Bitfield enums should be able to use #[repr(transparent)] instead of #[repr(C)] when available.

This should be a straight-forward change in src/codegen/mod.rs. Also rust version should be taken into account, see src/features.rs.

[highfive]

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[LegNeato]

@highfive: assign me

[highfive]

Hey @LegNeato! Thanks for your interest in working on this issue. It's now assigned to you!