Use "ASET (a, i, v)" rather than "AREF (a, i) = v".

Use "ASET (a, i, v)" rather than "AREF (a, i) = v".

This how ASET and AREF are supposed to work, and makes
it easier to think about future improvements.  See
<http://lists.gnu.org/archive/html/emacs-devel/2012-08/msg00026.html>.
* charset.h (set_charset_attr): New function.
All lvalue-style uses of CHARSET_DECODER etc. changed to use it.
* lisp.h (ASET): Rewrite so as not to use AREF in an lvalue style.
(aref_addr): New function.  All uses of &AREF(...) changed.
(set_hash_key, set_hash_value, set_hash_next, set_hash_hash)
(set_hash_index): New functions.  All lvalue-style uses of
HASH_KEY etc. changed.
* keyboard.c (set_prop): New function.  All lvalue-style uses
of PROP changed.