Revert "Use 64-bit register values (#72)"

[cadmic]

I chose this implementation:

Although the MIPS ISA says that 32-bit ops should produce sign-extended 32-bit values in 64-bit code, it's faster if we let the high bits be garbage instead.

but it seems like strip and probably other 64-bit programs mix 32-bit and 64-bit instructions freely and I'm kinda worried this will cause problems. Now I think it would be better to keep using uint32_t for 32-bit-only programs, and use uint64_t + sign extension everywhere for 64-bit programs.

Probably some of the changes could be kept but I think a clean revert is easier to review.