define and load classes in memory with gen-class

Currently gen-class only works while AOT compiling, but just evaluates to nil while JIT loading.
The reason for this behaviour is historical and no longer relvant since landed in 1.7, which fixed the dynamic classloader definition issues and also made this exact same change for gen-interface. The only reason why this wasn't also done for gen-class is that I forgot about it.

This patch fixes this inconsistency

Patch: 0001-CLJ-2343-define-and-load-class-while-JITing-gen-clas.patch