Remove my_wcswidth() in favor of fish_wcswidth()

my_wcswidth() was just a wrapper around fish_wcswidth() already.
Instead, add two convenience overrides of fish_wcswidth() to common.h
that make it a drop-in replacement for my_wcswidth().
Make escape() return a wcstring

This avoids the potential for leaking the resulting string.
Escape the error string in process expansion errors

This prevents `echo %*` from printing a private-use character in the
error string.

Fixes #1720.