Regression fix Issue 11591 - std.typecons.Tuple -s with classes fails at runtime as associative array keys

This pushes a runtime error to a compile time error. It therefore has the potential to break existing code, but I suspect any such code was already broken (like the test12.d test case, see below).

The new error message also helpfully gives the correct form for the required opCmp function.

https://d.puremagic.com/issues/show_bug.cgi?id=11591