Add return stack fuckery: `>R R> R@ I J`

See section "The Return Stack" of
<http://www.forth.com/starting-forth/sf5/sf5.html>.

Note that J, in particular, is redefined to return the second value from
the return stack rather than the third -- our implementation does not
use the return stack to hold JMP targets (instead relying on the
Python call stack).