fix(typechecker): less permissive equality for optional types

[anton-trunov]

Closes #649

• [x] I have updated CHANGELOG.md
• ~[ ] I have documented my contribution in Tact Docs: https://github.com/tact-lang/tact-docs/pull/PR-NUMBER~
• [x] 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
• [x] I have run all the tests locally and no test failure was reported
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• [x] I did not do unrelated and/or undiscussed refactorings

[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