feat(engine) Rewrite control flow inside loops.

[maximebuyse]

Closes #393.

Loops are already translated as folds. This PR wraps their accumulators to add control flow information (in case of break return or continue). Some specific fold operators are also added to handle these wrapped accumulators.

This PR puts LocalMutation before RewriteControlFlow. In RewriteControlFlow, we now deal with the body of loops (to push control flow in return position, like this phase already does for returns outside loops). We also wrap loops that have a return inside a pattern matching over the result (to propagate a returned value), and apply the phase to inline what comes after in the branches of the pattern matching. Phase DropNeedlessReturns is now DropREturnBreakContinue. Inside loops it replaces return , break and continue by their encoding in the ControlFlow enum. It treats a value in return position of a loop body like a continue. Phase FunctionalizeLoops selects the correct fold operator (each fold should now come in 3 versions: the normal one, the one supporting break and continue, and the one supporting all type of control flow).

[maximebuyse]

Code generation should be almost stable. The remaining part is to write the specific folds as part of the fstar Rust_primitives

[maximebuyse]

@W95Psp still testing but you can start to review.

[W95Psp]

Thanks! I have a first set of comments!

[maximebuyse]

@W95Psp I think I fixed everything according to your feedback!

[W95Psp]

Great, looking at it. Next PR can you resolve discussions that you think are handled?