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Based on config define.

Useful when we have pointer to char inside string, but need to return char
index. (E.g. str.find()).

It defines types used by all other headers.

Fixes #691.

We still have that char vs byte dichotomy, but majority of string operations
now use byte.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.

Eg, in CPython stdlib, email/header.py has a form-feed character.