fcr.el: Add support for mutable slots

fcr.el: Add support for mutable slots

* lisp/emacs-lisp/fcr.el (fcr--defstruct-make-copiers): Adjust for the
case of mutable slots.  Optimize the mandatory arg case.
Don't mark the copiers as inlinable.
(fcr-defstruct): Allow `:type` and `:mutable` properties on slots.
(fcr--lambda): Add `mutables` arg.
(fcr-lambda): Pass it.
(fcr--copy): Add `mutlist` arg.
(fcr--get): Add `mutable` arg.
(fcr--set): New function.
(fcr--mut-getter-prototype, fcr--mut-setter-prototype):
New prototype functions.

* test/lisp/emacs-lisp/fcr-tests.el (fcr-test, fcr-tests):
Add test for copier with mandatory arg.
(fcr-test-mut, fcr-test--mutate): New test.

* lisp/emacs-lisp/nadvice.el (advice): Use separate copiers
for the two use-cases, to avoid relying on CL keywords, since they're
not optimized away via inlining any more.
(advice--make, advice--tweak): Adjust accordingly.