Support a new ["..."] key binding syntax

Support a new ["..."] key binding syntax

* doc/lispref/keymaps.texi (Key Sequences):
(Changing Key Bindings): Document the various key syntaxes.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-define-key)
(byte-optimize-define-keymap)
(byte-optimize-define-keymap--define): New functions to check and
expand ["..."] syntax at compile time.

* src/keymap.c (Fdefine_key): Understand the ["..."] syntax.
(syms_of_keymap): Define `kbd' symbols.