Multiply-declared definitions are not checked directly

[robsimmons]

In current dusa and rewrite branch dusa, this passes typechecking and then raises a runtime exception:

#builtin NAT_SUCC succ
#builtin NAT_SUCC s
b (s (s (s succ))).

Ideally we'd allow this, I think? But still raise an error when z is not used with the correct arity.