With this change, any (%bind) returned from (simplify-let) or (simplify-form)
can be assumed to be flat, with unique bound symbols. Before, this was only
true of (%lambda) forms and the output of (compile).
Exception is (%tail-call ...) form, which is permitted simply because
(%tail-call ...) transfers control unidirectionally; the enclosing
(%set! ...) form wouldn't be run anyway.