1. BigIntValue in AElf.Types package.
2. NFT Contract.
3. IFeatureActiveService: Now we can set the active height for a new feature for AElf and judge to active this feature by injecting IFeatureActiveService. (The states are stored in Configuration Contract.)
4. Tune Token Contract to support allowance settings.
5. .netcore 3.1 -> .net 6.0, with reforamtting & cleanup code structure.