Allow functions to have vague data types.

[drwhut]

With the VM now storing the types alongside the data in the stack, we should now be able to make custom functions with vague data types.

Values would still have distinct types in the VM, it would just mean that the VM would decide how to use them depending on their types.

We could always optimise by having more specific opcodes if we know for a fact a given value will be a distinct type.