Construct a new keymap composed of MAPS and inheriting from PARENT.
When looking up a key in the returned map, the key is looked in each
keymap of MAPS in turn until a binding is found.
If no binding is found in MAPS, the lookup continues in PARENT, if non-nil.
As always with keymap inheritance, a nil binding in MAPS overrides
any corresponding binding in PARENT, but it does not override corresponding
bindings in other keymaps of MAPS.
MAPS can be a list of keymaps or a single keymap.
PARENT if non-nil should be a keymap.