Implement first instructions mfc0/sw/bne/sll

* The instruction decoding was based on the handy table in ps2tek [1]
while the instructions themselves were written based on the document
added.

* The implementation makes heavy use of bitfields and unions in C++ to
make accessing different bits/sections of registers easier and more
intuitive. You may also notice the frequent casting (uint32_t)(int16_t)
which might seem useless but is there to force the compiler to generate
instructions to sign extend the offsets.

[1] https://psi-rockin.github.io/ps2tek/#eeinstructiondecoding