Translate describe_map to Lisp

Translate describe_map to Lisp

Third step in converting substitute-command-keys to Lisp.

* lisp/help.el (describe-map): New Lisp version of describe_map.
(help--describe-map-compare, help--describe-translation)
(help--describe-command, help--shadow-lookup): New helper
functions for describe-map.
(help--keymaps-seen, help--previous-description-column): New
variables.
* src/keymap.c
(Fkeymap__get_keyelt): New defun to expose get_keyelt to Lisp.
(Fdescribe_map_tree_old, Fdescribe_map): Remove defuns.
(Fdescribe_vector_internal): New defun to expose describe_vector to
Lisp in a way usable by describe-map.
(syms_of_keymap): New defsubrs for Fkeymap__get_keyelt and
Fdescribe_vector_internal.  Remove defsubrs for Fdescribe_map_tree_old
and Fdescribe_map.  Remove 'help--keymaps-seen'.

* test/lisp/help-tests.el
(help-tests-substitute-command-keys/shadow): Extend test.
(help-tests-substitute-command-keys/test-mode)
(help-tests-substitute-command-keys/compare-all)
(help-tests-describe-map-tree/no-menu-t)
(help-tests-describe-map-tree/no-menu-nil)
(help-tests-describe-map-tree/mention-shadow-t)
(help-tests-describe-map-tree/mention-shadow-nil)
(help-tests-describe-map-tree/partial-t)
(help-tests-describe-map-tree/partial-nil): New tests.