Prepare the roadmap about our Solidity front end

[hydai]

It has been a long time since we freeze our milestones of solidity front end. The language spec is changed. Our solidity front end is based on solidity v0.6.6 release but we don't finish all of the language features in the v0.6.6.

It's time to pick up the solidity front end and schedule the milestones for the solidity front end.

Some actions are required:

1. Check and list the unimplemented language features.
2. Create a roadmap with several milestones about these unimplemented language features.