[TM-282] Cleanup StoreClass a bit

Description

Problem:

1. First of all, StoreClass has an obsolete statement that top-level storage is a pair of big_map and other fields. This restriction has been removed.
2. store*OpsTopLevelStorage kinda suggests that it should only be used for top-level storage, but I think I encountered a valid use-case when it is used for something else. Imagine you have Storage which has A inside and it has some big_map. It makes sense to use this store*OpsTopLevelStorage logic to define StoreHasSubmap for A, but A is not top-level storage. Same would happen for field if A contained B which in turned contained some useful field.
3. composeStoreFieldOps is exported, but composeStoreSubmapOps. I think there is no reason for inconsistency, especially now that big_maps can appear everywhere.

Solution:

1. Update obsolete comments.
2. Rename *TopLevelStorage to *Deeper.
3. Export composeStoreSubmapOps as well.

Related issue(s)

https://issues.serokell.io/issue/TM-282

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