Enumerated properties should have to/from ICU4C APIs

[Manishearth]

Our enumerated properties are opaque wrappers around integers, using the same values used by ICU4C.

It's reasonable to abstract away our internal implementation, but the ICU4C values are pretty standard across i18n code, and it's reasonable to wish to access these values.

We should offer to/from u8 (or u16) APIs.

I propose we name them to_icu4c_value() and from_icu4c_value(), linking to ICU4C docs. There is no fallibility, unknown values are accepted.

I am not opposed to just having Into/From and documenting the impl as stably producing ICU4C values.

cc @sffc @echeran @robertbastian

[sffc]

I'm fine adding these, since as you observed this is already part of our observable behavior via TrieValue. Prefer explicitly-named functions as usual.

Perhaps to_icu4c_int or to_icu4c_discriminant?