stri_cmp* now do not allow for passing opts_collator=NA.
     From now on, stri_cmp_eq, stri_cmp_neq, and the new operators
     %===%, %!==%, %stri===%, and %stri!==% are locale-independent operations,
     which base on code point comparisons. New functions stri_cmp_equiv
     and stri_cmp_nequiv (and from now on also %==%, %!=%, %stri==%,
     and %stri!=%) test for canonical equivalence.