Avoid Lisp_Misc allocation if C stack suffices

Avoid Lisp_Misc allocation if C stack suffices

* src/fileio.c (union read_non_regular): New type.
(read_non_regular, Finsert_file_contents):
Use it to avoid	allocating a Lisp_Misc.
* src/keymap.c (union map_keymap): New type.
(map_keymap_char_table_item, map_keymap_internal):
Use it to avoid	allocating a Lisp_Misc.