Change _str suffix

[P-p-H-d]

The str suffix in M*LIB is used in two different contexts: either to identify a constant C string (const char *) or a formatted string (like in OUT_STR / IN_STR ...). This is confusing for the user. As such, it may be better to use the suffix _cstr for constant C strings and _fstr for formatted string. Need to mature before implementation. See also #82 for some discussion.

[P-p-H-d]

Change from _str to _cstr has been done.

[P-p-H-d]

I postpone change to _fstr. _cstr will be good enough.