Use uint256.Int rather than common.Hash for storage values to reduce memory...

Use uint256.Int rather than common.Hash for storage values to reduce memory allocation in opSload & opSstore (#575)

* Produce less garbage in GetState

* Still playing with mem allocation in GetCommittedState

* Pass key by pointer in GetState as well

* linter

* Avoid a memory allocation in opSload

* Use uint256.Int rather than common.Hash for storage values to reduce memory allocation in opSload & opSstore

* linter

* linters

* small clean up