doc: explain the two-adress scheme

compared to a stack-based VM with no args, and a typcial three-address register
machine with possible SSA transformation for a static optimizable compiler
oo: add isa? and subclass? methods

Warning: subclass? just walks a single inheritence list yet,
This will miss some mixin combinations, multiple inheritance (e.g. perl @ISA)
Need to implement Packed Vector Encoding (http://www.eecs.berkeley.edu/~jrb/pve/)
re-enable test/classes/creature.pn
assign: document plan in core/compile.c AST_ASSIGN lhs EXPR
assign: generaic ast walker and walk_msg callback

for now only:
  detect msg in rhs of assign to check if compile-time matching
  can be done. else defer to runtime matching, resp. sequential assignment.
assign: add quoted msg test, see returned bindings
assign: fix walker, add equal cmp with lhs value (WIP)

replace 1 = x with 1 equal x. not constant folding yet for such binops.
need to add a EQUAL op, which is better than CMP: for all objects check cmp == 0.
assign: add EQUAL op, compare any value, not just numbers

also added potion_table_equal, potion_tuple_equal, potion_any_equal, potion_table_cmp
assign: add testcases (no compile-time match yet)
equal jit: forgot about cl at 1