fix: as coins map value serialization type now generates correct code

[Gusarich]

Issue

Closes #985.

Checklist

• 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]

will add tests in a moment

[anton-trunov]

What about the FunC part, shouldn't it use load_coins in this case?

[Gusarich]

What about the FunC part, shouldn't it use load_coins in this case?

at first I thought that the FunC part works fine already, but just looked through the generated code for as coins maps and turned out that it's completely wrong and just uses 257-bit long integers. I will fix that too

[Gusarich]

but it'll be easier to fix after #941 gets merged

[anton-trunov]

Awesome stuff! Just minor comments