librustls: replace u32_enum_builder macro

We only used this for `rustls_result`, and in practice it only saved us
having to implement `From<u32>` by hand. That's handy, but comes at the
cost of needing to use the nightly toolchain w/ cbindgen so it can
expand the macro before generating the `.h`. That wasn't the worst, but
a nighty regression that has remained unfixed for months broke this
workflow.

In sum: life will be easier without the macro. RIP.

Author: cpu