Historically fish has used the functions `fish_wcstol`, `fish_wcstoi`, and
`fish_wcstoul` (and some long long variants) for most integer conversions.
These have semantics that are deliberately different from the libc
functions, such as consuming trailing whitespace, and disallowing `-` in
unsigned versions.
fish has started to drift away from these semantics; some divergence from
C++ has crept in.
Rename the existing `fish_wcs*` functions in Rust to remove the fish
prefix, to express that they attempt to mirror libc semantics; then
introduce `fish_` wrappers which are ported from C++. Also fix some
miscellaneous bugs which have crept in, such as missing range checks.
