Merge cc-experiment into main?

[odersky]

I wanted to get some discussion going when we should merge cc-experiment into main. All user-visible changes of cc-experiment should be controlled by flag -Ycc, and all library additions should be @experimental.

Once that's the case, I believe it's time to merge cc-experiment.

The upsides of doing this include:

• No more large scale risky rebases (it's been very hard at times to rebase such a large change set)
• Better visibility when changes to main break some aspect of cc-experiment
• Easier context switches
• I am the main contributor on both branches and this will save me a lot of time.
• It's a necessary step to allow experimental extensions such as safe exceptions
• It's a necessary step to use capture checking in the compiler itself.

The downsides include:

• Code in cc-experiment might be less stable than main
• Code in cc-experiment might make it harder to understand the compiler codebase (although it is fairly well factored out).