Properly nest all Loop-EndLoop pairs. Fixes #16.

Also fixes #18.  After all other Loop-EndLoop expansion is done, step
through the VLIW array from beginning to end.  Each Loop command pushes
its address to the top of a stack, and each EndLoop pops an address from
the stack.  This should give proper nesting behavior.

Error checking performed:
Loop without matching EndLoop
EndLoop without matching Loop
Total loop nesting depth > 8 (PulseBlaster max)