Build the code that applies DO to PLACE.
PLACE must be a valid generalized variable.
DO must be a function; it will be called with 2 arguments: GETTER and SETTER,
where GETTER is a (copyable) Elisp expression that returns the value of PLACE,
and SETTER is a function which returns the code to set PLACE when called
with a (not necessarily copyable) Elisp expression that returns the value to
set it to.
DO must return an Elisp expression. (fn PLACE DO)