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.
The original implementation without the test took me 3 hours (first time
seriously looking into this)
The functions take "wcharz_t" for smooth integration with existing C++ callers.
This is at the expense of Rust callers, which would prefer "&wstr". Would be
nice to declare a function parameter that accepts both but I don't think
that really works since "wcharz_t" drops the lifetime annotation.
This adds an implementation of fish_wcstoi in Rust, mirroring the one in
fish. As Rust does not have a string to number which infers the radix
(i.e. looks for leading 0x or 0), we add that manually.
