[#346] Added `checkStorage'` to read storages with big_maps

Description

Problem: In Morley.Nettest, the only way to check a contract's storage is to use checkStorage. However, this function can only retrieve storages if they don't contain big_maps (due to a limitation in the Tezos RPC).

Solution: In the future, we'll want to come up with a more permanent solution that works on both the emulator and a real network. But, for now, we added a checkStorage' function that works only on the emulator and is able to retrieve any storage.

Related issue(s)

Resolves part of #346 (closed)

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