GXemul Code2

Reimplementing the mak and mak_imm M88K instructions.

Reimplementing the M88K and/mask/xor instructions.

Reimplementing the M88k cmp and cmp_imm instructions.

Reimplementing the M88K br, or, and or.u instructions.

Continuing on write handlers for variables (root.step),
beginning on component tree snapshotting (using Clone). The
backward-step command now works reasonably well (but the only
snapshot taken so far is at step=0).

M88K model can now also be set.

Most likely, iirc, RAM begins at offset 0 in the O2.

MIPS zr register is now read-only, the MIPS "model" variable can be changed (and m_type is internally updated correctly).

Beginning on variable write handlers.

Merging in HISTORY file entries from the C++ branch into the main HISTORY file, for preservation.