Use AREF, ASET and ASIZE macros.

Use AREF, ASET and ASIZE macros.
(Fmake_sparse_keymap): Docstring fix.
(synkey): Remove.
(shadow_lookup): Move up.
Handle the case where lookup-key returns an integer.
(where_is_internal_1): Drop arg `keymap'. Don't check shadowing.
(where_is_internal_2): Adapt to fewer args for where_is_internal_1.
(Fwhere_is_internal): Allow `xkeymap' to be a list of keymaps.
Simplify/rewrite the keymap-finding code.
Add check for command shadowing, using shadow_lookup.