&get_indexsubitem: This now has a chance of working, thanks to:

&do_cmd_setindexsubitem:  New function.  Set the indexsubitem value from
	\setindexsubitem{(...)}.

&do_env_opcodedesc:  By default, don't index byte codes.

$INDEX_OPCODES:  New flag.  If true, index the byte codes.  Default is off.

Normalize indentation to 4 spaces everywhere.

Minor nits.