variable: cleanup to use rb_const_lookup

* variable.c: cleanup to use rb_const_lookup
* vm_insnshelper.c: ditto

This reduces casting and long lines.  This should make it easier to
switch to alternatives to st for constant storage.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47059 b2dd03c8-39d4-4d8f-98ff-823fe69b080e