expr: use mcel

expr: use mcel

The mcel API is simpler and corresponds more closely to how
Emacs etc. behave when the input has encoding errors,
since it treats each encoding-error byte separately.
* bootstrap.conf (gnulib_modules): Add mcel.
* src/expr.c: Include mcel.h instead of mbuiter.h.
(mbs_logical_cspn, mbs_logical_substr, mbs_offset_to_chars):
Use mcel API.
(mbs_logical_substr): Use ximemdup0 so as not to waste memory in
the result, fixing a FIXME.