feat: fromCell method for maps

[Gusarich]

Issue

Closes #1229.

Checklist

• I have updated CHANGELOG.md
• I have documented my contribution in docs/ and made the build locally
• 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]

1. Let's change the syntax here to make it consistent with fromCell for structs/messages.

2. We need negative tests showing what happens when the cell does not represent a valid map. We need to validate the cell and it seems to me we could use TVM's LDDICTS instruction here and check the remainder is an empty slice. Or is there a better approach? What do you think?

[anton-trunov]

hmmm, this should be consistent with what we have for structs:

struct Fizz { foo: Int }
let fizzCell = ...
let parsedFizz: Fizz = Fizz.fromCell(fizzCell);

So I would expect something like:

let fizz = map<Int, Int>.fromCell(cell);

This makes parsing harder but it seems it's too late to do it differently

[Gusarich]

2. We need negative tests showing what happens when the cell does not represent a valid map. We need to validate the cell and it seems to me we could use TVM's LDDICTS instruction here and check the remainder is an empty slice. Or is there a better approach? What do you think?

@anton-trunov LDDICTS doesn't check anything, and there are no TVM instructions for checking the whole dictionary for "correctness" actually. Implementation would have to iterate over the whole tree, which is very gas-consuming.

[anton-trunov]

@Gusarich The TVM docs say this for LDDICTS:

Loads (parses) a (Slice-represented) dictionary s' from Slice s, and returns the remainder of s as s''

So, if a cell is map, then the remainder should be an empty slice, if I understand this correctly.

[Gusarich]

So, if a cell is map, then the remainder should be an empty slice, if I understand this correctly.

@anton-trunov LDDICTS doesn't even check the header of the map, it just checks the one single bit indicating whether it's empty or not when it's represented as Maybe X (similarly to what .load_dict() does.

https://github.com/ton-blockchain/ton/blob/ea0dc161639ef2640876d6de06f7224ac5873847/crypto/vm/dictops.cpp#L121-L146