feat(gas): do not store self-code in dictionary of child contract codes

[anton-trunov]

This should reduce forward fees for Jettons, for instance.

Another possible optimization would be to remove the child contract dictionary for the common case when there are just one or very few child contracts and use cell references to store the child contract codes. But let's do it as a separate PR so we can check its impact too.

Issue

Closes #1012.

Checklist

• I have updated CHANGELOG.md
• I have added tests to demonstrate the contribution is correctly implemented: it reduces gas consumption for the common cases, makes storage improvements, etc. and does not have too much negative impact for the rest of the use-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]

Looks good, but for some reason it increases fees for some transactions in initof tests 🤔 we definetely need to benchmark this with https://github.com/tact-lang/jetton

[anton-trunov]

for some reason it increases fees for some transactions in initof tests

do you still have the exact numbers?

because it definitely decreased gas in the benchmarks:

[anton-trunov]

Nice results! Let's merge