bytecomp.el: Rewrite the way we print dynamic docstrings

bytecomp.el: Rewrite the way we print dynamic docstrings

We used to print dynamic docstrings "manually" for two reasons:

- References should look like `(#$ . POS)` but `prin1` was unable
  to print just `#$` for an sexp.
- `make-docfile` needed to find those docstrings and the object
  to which they belonged.

The second point is moot now that we don't use `make-docfile` on
`.elc` files.  So this patch lifts the first restriction,
using `print-number-table`.

The rest of the patch then simplifies and regularises the
bytecompiler's generation of dynamic docstrings, which can
now also easily be done for "inner" defvars and other places.

* src/print.c (print_preprocess, print_object): Handle strings in
`print-number-table`.
(Vprint_number_table): Improve docstring.

* lisp/emacs-lisp/bytecomp.el:
(byte-compile--list-with-n): New function.
(byte-compile--docstring-style-warn): Rename from
`byte-compile-docstring-style-warn` and change calling convention.
(byte-compile--\#$, byte-compile--docstrings): New vars.
(byte-compile-close-variables): Bind them.
(byte-compile--docstring): New function.
(byte-compile-from-buffer): Set `byte-compile--\#$`.
(byte-compile-output-file-form): Use `byte-compile--\#$` instead
of special casing specific forms.
(byte-compile--output-docform-recurse, byte-compile-output-docform):
Delete functions.
(byte-compile-file-form-autoload, byte-compile-file-form-defalias)
(byte-compile-file-form-defvar-function, byte-compile-lambda):
Use `byte-compile--docstring` and `byte-compile--list-with-n`.
(byte-compile--declare-var): Add optional `not-toplevel` arg.
(byte-compile-defvar): Add `toplevel` arg.  Use `byte-compile--docstring`.
(byte-compile-file-form-defvar): Delegate to `byte-compile-defvar`.
(byte-compile--custom-declare-face): New function.  Use it for
`custom-declare-face`.
(byte-compile-file-form-defmumble): Use `byte-compile-output-file-form`

* src/doc.c (Fdocumentation_stringp): New function.
(syms_of_doc): Defsubr it.
(store_function_docstring): Remove left-over code from when we
used DOC for the docstring of some Lisp files.

* lisp/cus-face.el (custom-declare-face): Accept dynamic docstrings.
* lisp/faces.el (face-documentation): Handle dynamic docstrings.
* lisp/help-fns.el (describe-face): Simplify accordingly.

(cherry picked from commit e9a668274e441645aed28e8c353187dfed35fcae)