[TM-276] Safe UStore migrations (declarative approach)

Description

This MR implements a prototype of type-safe UStore migration using a declarative approach.

It means that for each field of new UStore version you supply a way to create that field from old storage. Columnars magic is used to use the same UStore template datatype for UStore initializer (in mkUStore function) and migration specification. Unnecessary fields will be removed automatically.

This MR also contains a bunch of helpers which can be useful in other cases, for migration stuff please see only [TM-276] Add migration commit.

Also see an alternative approach in !50 (merged).

TODO: remove fields automatically (this requires some copy-paste from !50 (merged) which I will do if that MR gets closed).

Related issue(s)

https://issues.serokell.io/issue/TM-276

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