The nix crate had all its default features enabled, which included features that
are not present under BSD. We should only enable the select subset of crate
features that we know are available cross-platform (or else use conditional
targeting in Cargo.toml to only enable Linux-only features when compiling for
Linux targets).
For now, it seems we can just use the nix crate with all features disabled as it
still builds under Linux and FreeBSD in this state.
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.
