first loop in the vm!

I've implemented enough instructions in the VM that I can now do a loop (a.k.a. a tail-recursive function call).

I still need to work on the VM insns, there's a lot of allocation taking place that shouldn't... also, there's range-checking on each and every vector access.. even including stuff like the register file. Given those caveats, it's only about 4X slower than a similar python loop. I'm pretty sure I should be able to make it faster than python, we'll see.

--- cps ---
   0 = new_env [] 1
   -   push_env [0] None
   1 = close [] 'loop_0'
       0 = lit [] 3
       1 = varref [] ((0, 0), False, {n_1})
       0 = primop [0, 1] ('%=',)
       0 = test [0] None
           0 = lit [] 1
               return [0] None
           0 = varref [] ((0, 0), False, {n_1})
           1 = lit [] 2
           0 = primop [0, 1] ('%-',)
               tr_call [0] (1, )
           return [0] None
   -   store_tuple [1, 0] (0, 1, 1)
   0 = new_env [] 1
   1 = lit [] 0
   0 = store_tuple [1, 0] (0, 1, 1)
   1 = varref [] ((0, 0), True, {loop_0})
       invoke_tail [1, 0]