Reduce code duplication in parts of (auto)load&defalias

Reduce code duplication in parts of (auto)load&defalias

* src/data.c (defalias): New function, extracted from `Fdefalias`.
(Fdefalias): Use it.
(Ffset): Don't handle `Vautoload_queue` here, handle it in
`defalias` instead.
* src/comp.c (comp--register-subr): Use `defalias` instead of
duplicating its code.

* src/eval.c (load_with_autoload_queue): New function, extracted from
`Fautoload_do_load`.
(Fautoload_do_load): Use it.
(un_autoload): Mark it as static.
* src/fns.c (Frequire): Use it as well.

* src/lisp.h (defalias, load_with_autoload_queue): New declarations.
(un_autoload): Remove declaration.