Implement fromCell and fromSlice methods for structs

[Gusarich]

Closes #344

• I have updated CHANGELOG.md
• 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

[Gusarich]

Just realised that I didn't add any negative tests for struct parsing

[anton-trunov]

We need some tests to make sure the following works as expected.
First, the user defines a fromCell:

extends mutates fun fromCell(self: Cell, _: Cell): MyStruct1 {
  return MyStruct1 { ... };
}

And now the user is trying to do something weird:

contract StructsTester {
    receive() { }

    get fun fromCell1(src: Cell): MyStruct1 {
        let MyStruct1: Cell = beginCell().endCell();  // it's a cell here!
        let s: MyStruct1 = MyStruct1.fromCell(src);
        return s;
    }
}

What do you expect to happen in this case?

[Gusarich]

@anton-trunov do you mean a user-defined function with the same name - fromCell or fromSlice?

I think it should work the same way as it works for all other built-ins such as toCell for structs, and set, get and del for maps.

[anton-trunov]

@Gusarich Yep. Because right now this shadowing is allowed and it overrides the generated code.

[Gusarich]

@Gusarich Yep. Because right now this shadowing is allowed and it overrides the generated code.

Should we forbid such shadowing for all built-in functions?

UPD: oh, we actually wanted to do this already, but only implemented throwing errors for global.ts: #351

[anton-trunov]

Yeah, we do not allow any form of shadowing in Tact. And if we do, it's a bug

[anton-trunov]

btw, there is some merge conflicts

[Gusarich]

Yeah, we do not allow any form of shadowing in Tact. And if we do, it's a bug

In this case maybe we should create a separate issue about that and resolve that in another PR?

[anton-trunov]

In this case maybe we should create a separate issue about that and resolve that in another PR?

Yeah, there are issues like that, for instance #307. There is a special tag for those scope: var-scope Scoping rules for identifiers

[anton-trunov]

LGTM! Just some minor tweaks in the suggested changes