Properly scoped and selected the import of std.traits in std.ascii, so that it is only imported for the 2 functions that use it, toLower and toUpper.

As I don't have access to a system able to build the unittests, these are the only changes I'm willing to make, but do prevent std.traits from being imported when toLower and toUpper aren't referenced.

For the moment there will still be an unscoped import for std.traits in the unittest build.