Emit byte-codes to unbind the variables bound by CLAUSES.
CLAUSES is a `let'-style variable binding list. INIT-LEXENV should be a
lexical-environment alist describing the positions of the init value that
have been pushed on the stack. If PRESERVE-BODY-VALUE is true,
then an additional value on the top of the stack, above any lexical binding
slots, is preserved, so it will be on the top of the stack after all
binding slots have been popped.
(fn CLAUSES INIT-LEXENV &optional PRESERVE-BODY-VALUE)