Partial fix for issue# 9769.

This makes it so that the global opEquals function is templated. It is
therefore now possible for classes to implement opEquals without
overiding Object's opEquals method, including giving it different
attributes (such as const).

However, the hack version of the global opEquals which takes const
Objects and casts away const has been left for the moment, and the
opEquals on Object has not been touched. At minimum, before the hack
version can go away, work will need to be done on the other classes in
object_.d to make them handle const properly. Also, the opEquals on
Object will probably have to go away before that if we don't want to
break code, as it's the only reason that const Objects have been able to
be compared before now, and until types stop using Object's opEquals,
that's still the only way that they'll be able to be compared. But at
least with these changes, it'll be possible for classes to be compared
without it if they change their signatures for opEquals to not take
Object.

http://d.puremagic.com/issues/show_bug.cgi?id=9769