Change string-match and string-search to require NFC inputs.

Change string-match and string-search to require NFC inputs.

This is because comparison requires that the strings be in the same
normalization form, and these procedures return indices into the strings.  We
can't normalize them after the call, because then the returned indices will
refer to strings that are potentially different from the arguments.

Since nearly all strings are in NFC by default this should not be a serious
drawback.

Additionally, the -ci versions of these procedures have been eliminated,
basically for the same reason.  If the caller needs that functionality they
should call string-foldcase themselves.

Note that this doesn't affect comparisons that don't return indices.