Copenhagen 0.1 release

[liufengyun]

• [x] separate typed trees and untyped trees and make monadless work for the new design (difficult)
• [x] solve the path-dependent problem using the reflection trick
• [x] rebase compiler fork and make upstream PR to Dotty
• [x] implement more macros, testing and add more APIs
• [x] improve experience with quasiquotes -- auto lifting
• [ ] better documentation

4 weeks to go, all pull requests and help are highly appreciated.