multiple ruleset support

[fmixolydian]

Many assemblers (e.g. CA65) have a feature where you can switch between different CPU architectures within the same assembler file, e.g. for writing code for coprocessors.

It would be nice if customasm had a feature where you could choose which ruleset to enable. For example:

#ruledef isa_a {
  mov r{a: u4}, r{b: u4} => 0x10 @ a @ b
  switch_to_b => 0x20
}

#ruledef isa_b {
  add r{a: u4}, r{b: u4} => a @ b
  switch_to_a => 0xFF
}

#arch isa_a
mov r0, r6
mov r7, r2
switch_to_b

#arch isa_b
add r6, r7
add r8, r1
switch_to_a