This also cleans up and removes unnecessary usage of FFI-oriented `feature_metadata_t`,
which is only used from Rust code after `builtins/status` was ported.
We should only be dealing with wcharz_t at the language boundary.
Rust callers should prefer the equivalent &wstr.
Since wcsfilecmp() is no longer exposed directly it can take &wstr only.
This is early work but I guess there's no harm in pushing it?
Some thoughts on the conventions:
Types that live only inside Rust follow Rust naming convention
("FeatureMetadata").
Types that live on both sides of the language boundary follow the existing
naming ("feature_flag_t").
The alternative is to define a type alias ("using feature_flag_t =
rust::FeatureFlag") but that doesn't seem to be supported in "[cxx::bridge]"
blocks. We could put it in a header ("future_feature_flags.h").
"feature_metadata_t" is a variant of "FeatureMetadata" that can cross
the language boundary. This has the advantage that we can avoid tainting
"FeatureMetadata" with "CxxString" and such. This is an experimental approach,
probably not what we should do in general.
