enc/unicode.c: constify

* enc/unicode.c (code{2,3}_{cmp,hash}): constify and adjust
  argument types.

* enc/unicode.c (onigenc_unicode_fold_lookup): constify.

* enc/unicode.c (onigenc_unicode_apply_all_case_fold): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46056 b2dd03c8-39d4-4d8f-98ff-823fe69b080e