Optional types in type parameters
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The easiest solution is probably to just require either:
1) both the parameter declaration and the usage to be optional, or
2) none of them to be optional.

For example:

    type List<ref T> = ...
    type Slot<ref? T> = ...

    func f(ref List<ref Thing> l)
    func g(ref Slot<ref? Thing> s)

How about merging with word-sized or smaller types?
- Merging should ONLY be allowed for non-optional types