feat: map.deepEquals for deep map comparison

[Gusarich]

Closes #196

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

[anton-trunov]

Let's also add a negative test case based on #196 (comment). It should show that deepEquals works fine on it, whereas == fails.

[Gusarich]

We should also note in documentation that this function is very expensive in terms of gas (iterating a map + using slice_hash, both are expensive things), and in most cases it'll be enough to use hash comparison via ==.

[anton-trunov]

Awesome work!