fix(typechecker): less permissive equality for optional types

[anton-trunov]

Closes #649

• I have updated CHANGELOG.md
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• I have added tests to demonstrate the contribution is correctly implemented: this usually includes both positive and negative tests, showing the happy path(s) and featuring intentionally broken cases
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[Gusarich]

Why do we forbid comparing X and X?? It makes sense to allow comparing these two. Generated code would look like this:

1. First we check whether X? is null and return false in this case because null cannot be equal to X
2. If X? is not null, we can just compare them as X and X

Same for comparison of X? and X?: First we check if either both of these are null or both of these are not null and compare them accordingly, and in other cases (first is null and second is not or vice versa) we just return false

[anton-trunov]

First we check whether X? is null and return false in this case because null cannot be equal to X

In general, equality comparisons only make sense for terms of the same type. The check you describe should be done by the user, e.g. if (x != null) { return x!! == y } where, for instance, x: Int? and y: Int.

Same for comparison of X? and X?: First we check if either both of these are null or both of these are not null and compare them accordingly, and in other cases (first is null and second is not or vice versa) we just return false

Look at the tests: we don't forbid equality comparisons for X? and X? in general, just for non-equality types like Builder or structs.

[anton-trunov]

Why do we forbid comparing X and X?? It makes sense to allow comparing these two

wait, we don't actually forbid that -- we check assignability of types

so, my example above is not correct, because Int and Int? are both equality types

[anton-trunov]

@Gusarich Added a test for that: https://github.com/tact-lang/tact/pull/650/files#diff-570728cab178f5e1fa5f1885b32ee9826e95de3a357d341adbc331578591ab1a