Return FORM with all its lambdas changed so they are closed.
ENV is a lexical environment mapping variables to the expression
used to get its value. This is used for variables that are copied into
closures, moved into cons cells, ...
ENV is a list where each entry takes the shape either:
(VAR . (car EXP)): VAR has been moved into the car of a cons-cell, and EXP
is an expression that evaluates to this cons-cell.
(VAR . (internal-get-closed-var N)): VAR has been copied into the closure
environment's Nth slot.
(VAR . (apply-partially F ARG1 ARG2 ..)): VAR has been λ-lifted and takes
additional arguments ARGs.
EXTEND is a list of variables which might need to be accessed even from places
where they are shadowed, because some part of ENV causes them to be used at
places where they originally did not directly appear.
(fn FORM ENV EXTEND)