Improve `StorageContains`

Description

Problem: in the current implementation of StorageContains I can't write

type StorageC store meta = StorageContains store
  [ "field" := meta
  , ...
  ]

meaning

type StorageC store meta =
  ( StoreHasField store field meta
  , ...
  )

because with polymorphic meta, StorageContains cannot deduce which exactly case do we mean.

Solution: add a special case in the type family for such situations, so that "field" := Identity meta means StoreHasField store field meta.

Related issue(s)

Resolves #

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