Implement read-only `StorageState` functionality to translation shim type

[timwu20]

Issue Summary

From design doc:

The following StorageState methods are relatively easy to implement in the translation shim by utilizing db.Backend.
internal/client/db/backend.go

GetStorage(root *common.Hash, key []byte) ([]byte, error)
GetStorageByBlockHash(bhash *common.Hash, key []byte) ([]byte, error)
GetKeysWithPrefix(root *common.Hash, prefix []byte) ([][]byte, error)
GetStorageChild(root *common.Hash, keyToChild []byte) (trie.Trie, error)
GetStorageFromChild(root *common.Hash, keyToChild, key []byte) ([]byte, error)
GetStateRootFromBlock(bhash *common.Hash) (*common.Hash, error)

StorageRoot() (common.Hash, error) 

Entries(root *common.Hash) (map[string][]byte, error)

LoadCode(hash *common.Hash) ([]byte, error)
LoadCodeHash(hash *common.Hash) (common.Hash, error)

GetStorageXXX methods are easily accessible by translating and implementing the StorageProvider trait for Client internal/client/client.go

GetStateRootFromBlock can be retrieved from already introduced HeaderBackend interface which db.Backend implements. Same for StorageRoot.

Entries should be refactored to utilize an iterator where Entries is called. I propose refactoring this method to return a PairsIter which iterates through the keys and values.

LoadCode and LoadCodeHash should be trivially implemented by accessing the state trie for a given hash and retrieving the entry with key ":code".

Acceptance Criteria

• Introduce StorageProvider interface and implement it
• Implement GetXXX methods
• Implement StorageRoot method
• Revise Entries function call to return iterator. (removed from scope)
• Update all references to Entries to use iterator.
• Implement LoadCode and LoadCodeHash.
• Unit tests.