Add RewriteControlFlow phase.

[maximebuyse]

Closes #393

[maximebuyse]

@W95Psp I refactored like we discussed using Ast_utils.collect_let_bindings to treat directly with a sequence of "statements". This is indeed simpler and seems to work well.

[W95Psp]

Looks great to me, thank you!
I have one small code remark, but also, as we discussed yesterday, this phase sometimes adds extra let _ = () in. Can you try to spot where those are produced and prevent it?
For an example see https://hax-playground.cryspen.com/#fstar/595d6dddc9/gist=9c4e9735d9e4055b3407c81bc4238e8f

[maximebuyse]

Looks great to me, thank you! I have one small code remark, but also, as we discussed yesterday, this phase sometimes adds extra let _ = () in. Can you try to spot where those are produced and prevent it? For an example see https://hax-playground.cryspen.com/#fstar/595d6dddc9/gist=9c4e9735d9e4055b3407c81bc4238e8f

I fixed that. I noticed that in your example there is also a let _ = 3. Should we remove that as well?

[W95Psp]

Yeah, the 3 is useless, and since all side effects are moved away, I would say such values will always be useless.
However, since the user typed it, I'm in favor of keeping it: in general I think it's a great thing to keep user inputs, even if the input make no much sense.
So let's keep that as it is!

[W95Psp]

Thanks, I have one tiny tiny stupid remark (sorry for that 😅), and then let's merge it!