hash.c: refactor loop

* hash.c (rb_hash): rewrite with while instead of goto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44246 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
hash.c: warnings in rb_hash_reject

* hash.c (rb_hash_dup_empty): split from rb_hash_dup.

* hash.c (rb_hash_reject): add warnings (currently suppressed).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44247 b2dd03c8-39d4-4d8f-98ff-823fe69b080e