Add new Lisp implementation of substitute-command-keys

Add new Lisp implementation of substitute-command-keys

This is only the first step towards a full Lisp implementation, and
does not remove the old C code.  On the contrary, it is partly based
on using the old C code, which is to be replaced in steps.  This also
makes it easy to test that it produces the same output as the old.

* src/doc.c (Fsubstitute_command_keys_old): Rename from
Fsubstitute_command_keys.
(Fget_quoting_style): New defun to expose text_quoting_style to Lisp.
(syms_of_doc): Expose above symbols.
* lisp/help.el (substitute-command-keys): New Lisp version of
substitute-command-keys.  (Bug#8951)

* src/keymap.c
(Fdescribe_map_tree): New defun to expose describe_map_tree to Lisp.
(syms_of_keymap): New defsubr for Fdescribe_map_tree.

* src/keyboard.c (help_echo_substitute_command_keys):
* src/doc.c (Fdocumentation, Fdocumentation_property):
* src/print.c (print_error_message):
* src/syntax.c (Finternal_describe_syntax_value): Fix calls to use new
Lisp implementation of substitute-command-keys.

* test/src/doc-tests.el: Remove file.
* test/lisp/help-tests.el: Add tests for substitute-command-keys
copied from above file.