Add StoreSubmapOps drivers for working with new ManagedLedger

Description

Problem: recently we changed ManagedLedger implementation so that instead of one big_map with all the related values grouped it now has several separate big_maps, each containing only one value. In one of the projects, we would like to use the new implementation without changing the storage format because big_map migrations are difficult.

Solution: add implementations for StoreHasSubmap that handles the mentioned scenario.

This MR also brings several other changes that seem sweet to have.

Note: here I perform some changes over files that have been recently moved, will rebase on master once one another MR is merged.

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