Warn about missing backslashes during load

Warn about missing backslashes during load

* src/lread.c (load_warn_unescaped_character_literals, Fload, read1)
(syms_of_lread): Warn if unescaped character literals are
found (Bug#20152).
* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Check for
unescaped character literals during byte compilation.
* test/src/lread-tests.el (lread-tests--unescaped-char-literals): New
unit test.
(lread-tests--with-temp-file, lread-tests--last-message): Helper
functions for unit test.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--unescaped-char-literals): New unit test.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--with-temp-file):
Helper macro for unit test.