Implement pre-defining structs/unions

[vaartis]

When a struct or a union is already defined in a lib with the same name as the transformer would generate, use that defined structure instead of generating a new one. One particular usecase for this is defining structures that use bitfields manually and then using those instead of the incorrectly generated by crystal_lib.

This is somewhat related to #70, but even without that generating the same struct if one is already defined makes the file not compile, so using the one already defined seems like the proper behaviour.