Add MEMSWAP instruction

[ninezerozeronine]

Something like:

MEMSWAP [#15] [#155]
MEMSWAP [A] [B]

The temporary value can be stored in the ALU register by passing it straight through the ALU.

This would replace something like:

PUSH A
PUSH B
LOAD [#15] A
LOAD [#155] B
STORE A [#155]
STORE B [#15]
POP B
POP A

[ninezerozeronine]

This actually needs 2 spare registers. The "secret" ALU reg isn't enough.