Construct and return a new keymap, of the form (keymap CHARTABLE . ALIST).
CHARTABLE is a char-table that holds the bindings for all characters
without modifiers. All entries in it are initially nil, meaning
"command undefined". ALIST is an assoc-list which holds bindings for
function keys, mouse events, and any other things that appear in the
input stream. Initially, ALIST is nil.
The optional arg STRING supplies a menu name for the keymap
in case you use it as a menu with `x-popup-menu'. (fn &optional STRING)