Implement read/write operations and KUSEG regions

* Now that the BIOS is loaded we can start executing it!
The starting address the EE uses is 0xbfc00000 which maps
to KUSEG1. Since all KUSEG regions except KUSEG2 are mirrors
of each other we only need to translate the address to the
KUSEG appropriate.

* The functional differences between KUSEG0/1 are minimal and
very niche so I won't bother emulating them now. Address wise
we can notice that the only difference between addresses is the
most significant half byte. By using that byte as an index in
a mask table we can define an appropriate mask for each KUSEG
address. Idea taken from a very handy PSX document I discovered
last year [1]

[1] https://svkt.org/~simias/guide.pdf (43)