Implement btowc() in terms of mbrtowc() instead of sgetrune(), and
wctob() in terms of wcrtomb() instead of sputrune(). There should be
no functional differences, but there may be a small performance hit
because we make an extra function call.

The aim here is to have as few functions as possible calling
s{get,put}rune() to make it easier to remove them in the future.