Fix 2695: local open + let bindings

[SanderSpies]

Solves #2695.

Makes it possible to use:

 let y = Promise.Ops.(
  let* x = Js.Promise.resolve(42);
  Js.Promise.resolve(x * 2);
);

Printing isn't perfect, but good enough for now.

[anmonteiro]

This looks great, thanks!

[anmonteiro]

this looks exactly like the then branch.

I'm surprised this doesn't break let open Foo; bar; for example?

Perhaps the letop case should be added to isSeriesOfOpensFollowedByNonSequencyExpression, then we'd get printing for free?

[SanderSpies]

Well, not exactly the same. The first branch results into extra braces:

-      let* x = Js.Promise.resolve(42);
-      Js.Promise.resolve(x * 2);
+      {
+        let* x = Js.Promise.resolve(42);
+        Js.Promise.resolve(x * 2);
+      }

[SanderSpies]

I did make a small change to avoid code duplication

[anmonteiro]

I don't see how this is correct yet. I might be missing something.

Aren't we replacing ALL open calls with Foo.( ... )?

[SanderSpies]

@anmonteiro

Not exactly, but the solution wasn't perfect (it didn't work for nested local open modules). I've tried to improve on it with help of an extra attribute set by the parser.