It's not clear whether or not `system_wcwidth()` was picked solely because of
the namespace conflict (which is easily remedied) but using the most obvious
name for this function should be the way to go.
We already have our own overload of `wcwidth()` (`fish_wcwidth()`) so it should
be more obvious which is the bare system call and which isn't.
(I do want to move this w/ some of the other standalone extern C wrappers to the
unix module later.)
Pull in the correct descriptions merged from across the various C++ header and
source files and get rid of the getter function that's only used in one place
but causes us to split the documentation for FISH_EMOJI_WIDTH across multiple
declarations.
