Translate describe_map_tree to Lisp
Second step in converting substitute-command-keys to Lisp. * lisp/help.el (describe-map-tree): New Lisp version of describe_map_tree. (substitute-command-keys): Update to use above function. * src/keymap.c (Fdescribe_map): New defun to expose describe_map to Lisp. * src/keymap.c (syms_of_keymap): New variable 'internal--seen'; a temporary kludge planned for removal. New defsubr for Fdescribe_map. * test/lisp/help-tests.el: Minor cleanups.