* fns.c (validate_subarray): Rename from validate_substring,

* fns.c (validate_subarray): Rename from validate_substring,

since it works for vectors too.  New arg ARRAY.  Optimize for the
non-nil case.  Instead of returning bool, throw an error if out of
range, so that the caller needn't do that.  All uses changed.
Report original values if out of range.
(Fsubstring, Fsubstring_no_properties, secure_hash):
Also optimize the case where FROM is 0 or TO is the size.