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. The final change is to mark `toLower` and `toUpper` explicitly as `@safe pure nothrow`, as these are checked in unittests, but a unittest wouldn't tell you what was causing one of the attributes to not be valid for them. ~~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.~~

This was previously [Pull Request 1557](https://github.com/D-Programming-Language/phobos/pull/1557), but due to limitations of the Github web interface, I have to create a new pull request to compact the changes into one commit.