[#520] Allow retrieving a contract's storage as an expression in cleveland

Description

Problem: We need a way to get a contract's storage without knowing its type in cleveland by returning it as an Expression.

Solution: Add an abstract niGetStorageExpr to NettestImpl and call it in nettestCapImpl. Implement this new function in Client by changing niGetStorage to not convert it to an expression and rename to niGetStorageExpr. Create niGetStorage to use niGetStorageExpr, but call fromExpression now. In Pure, abstract the code to getting an AddressState's ContractState to a function and change niGetStorage to use this new function. Create niGetStorageExpr to use this function, but call toExpression.

Related issue(s)

Resolves #520 (closed)

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