Scoped the imports in std.ascii

This basically scopes the imports in std.ascii, as well as modifies those imports to be selective in what they import. There are also a couple of style changes, namely the ordering of values in an anonymous enum, and the use of only 1 indent level within an array declaration. As well as making a set of globals that were immutable into enums, which means they should now be fully accessible for CTFE. By making _ctype into an enum, all methods in std.ascii should now be CTFE'able.