Case-insensitive file paths

[Clownacy]

This one may seem esoteric, but the lack of it is currently preventing most of the tests from passing on Linux. Yes, this could be mitigated by just using a case-insensitive filesystem, but I don't think users would like being told that, so ideally ClownAssembler would emulate this internally.

This is probably best left until after the C++ migration, as directory iteration and the like is not easily to do portable in ANSI C.