The previous version of icetUnsafeStateGet blindly cast whatever
type was stored in the state variable to whatever the caller
wanted.  If the caller got the type wrong, that would be hard to
debug.  Replaced this method with several type-specific versions.
This makes a compile-type check to be sure the user is storing
the expected type into a compatible pointer and a run-time check
to make sure that the expected pointer is what is actually
stored.  If the run-time check fails, then a crash will probably
still occur, but it will be much easier to diagnose.