Added FS functions to Archive and Archive_RomFS

Added pack_u32 and unpack_u64 helper functions to math_util.h
This doesn't look like something which should be in Common, and even then not in MathUtil. Looks like it has a lot of potential for misuse. I'd just put it into the corresponding file you used it in. Also, it's not really clear what this should even be used for at all (if you need to read the definition of a "common" function to realize what it's useful for, it really isn't a good utility to put in the common library).