Better compilation of char-before, backward-char and backward-word

Better compilation of char-before, backward-char and backward-word

Implement char-before, backward-char and backward-word as compiler
macros instead of byte-compile handlers so that the source-level
optimiser gets to simplify the result.  In particular, this removes
some branches.

* lisp/emacs-lisp/bytecomp.el (byte-compile-char-before)
(byte-compile-backward-char, byte-compile-backward-word): Remove.
(bytecomp--char-before, bytecomp--backward-char)
(bytecomp--backward-word): New.