fish_wcstoi_partial is like fish_wcstoi: it converts from a string to an
int optionally inferring the radix. fish_wcstoi_partial also returns the
number of characters consumed.
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.