[TM-337] More type safety in migration framework

Description

The final set of adjustments for UStore migrations.

The main problem till now was that MigrationScript does not remember versions os storage it converts between, so the developer is restricted to construct a correct migration but still can apply it to a wrong version of the contract. Now we can prevent that, at least in tests.

Some solutions taken here might be controversial, so please express your opinion.

Related issue(s)

https://issues.serokell.io/issue/TM-337

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