format: Create a new "struct fmt_affix" for prefix and suffix strings.

This combines two changes: changing the string type for numeric
prefix and suffix strings from "struct substring" to plain "char *",
and putting the string inside a new structure.  Both of these will
make more sense after the following commit, which adds another member
to the new structure and stops using the length of the string in so
many places (which is the reason that "struct substring" was a good
choice).