Disallow non-invariant types in Pointer#value=

[asterite]

Fixes #6997

The code in #6997 shouldn't compile at all, as I explained in the issue.

This PR changes Pointer#value= to disallow non-invariant types. If you have a Pointer(Array(String | Int32)) then you must insert an Array(String | Int32) into it. You can't insert Array(String) or Array(Int32).

As a second step to this I would like to change all restrictions to always check invariance for generic types. Right now we can do x : Range(Int, Int) as a type restriction to say "any type that inherits Int" but that gives the problem of #6997. We can change the semantic to mean "must be an Int" (in this case it doesn't make sense) and eventually introduce some syntax to mean "must be an Int or inherit it".

[asterite]

We should probably not merge this because it breaks for tuples. That is, one should be able to do:

ptr = Pointer(Tuple(Int32 | String)).malloc(1)
ptr.value = {1}

That's because the compiler automatically expands "smaller" tuples to "bigger" tuples. But it doesn't do that for other types. So... we should change the matching semantic: for tuples it's fine to be covariant, for other types it's not.

[asterite]

(or we can remove that extra tuple logic, it's probably better to simplify the language)

[j8r]

@asterite I guess this compiles:

ptr = Pointer(Tuple(Int32 | String)).malloc(1)
ptr.value = {1.as(Int32 | String)}

I'm not sure Tuple with unions are as used as for Array