(state breaking) make get set in unorderedmap more efficient

[ailisp]

This is a state breaking change, addresses #107, inspired by @austinabell 's suggestions

I assume developers contract using unorderedmap is still in testnet / no production users and safe to clear the state to use the state breaking change. If this is not the case, then you can use the following strategy to upgrade state:

1. write a one-method contract with old near-sdk-js, iterate and get all key-value pairs in your unordered map
2. clear states that are prefixed by the old unordered map prefix
3. write a one-method temp contract with new near-sdk-js, add a method to add all (key, value) to the new unordered map
4. remove the temp contract, build your contract with new-sdk-js and deploy!

[volovyks]

Nice simplification!