Added FS functions to Archive and Archive_RomFS

Added pack_u32 and unpack_u64 helper functions to math_util.h
I was debating whether to put it in math_util or memory_util. I considered math because it had to do with combining integers, and memory because I could see those functions being used with memory operations. 

As for putting it in common, I think it has the potential to be used in several different files in the project. I think it could be useful beyond that use-case with the filesystem.