Add GetNativeFunction (bug 6200).

Two bug fixes here. First, the type system has to start enforcing functions == objects, which means they can't be illegally retagged. As a workaround for older code which might do `Function:GetNativeCell()`, I added `GetNativeFunction`.

The second fix is that using `<unknown>` as a placeholder name will assert in sc7.c since it doesn't expect weird characters. I changed it to `__unknown__` instead.