Pipe completion unknown/type parameter return types

[zth]

This fixes pipe completion for functions returning a type not known until the code has compiled. Array.reduce being an example, which has the signature ('a, ('a, 'b) => 'a) => 'a. It returns 'a but we don't know what 'a is unless it has compiled.

This makes it so that this now completes as int (after saving):

let _ = [123]->Belt.Array.reduce(0, (acc, curr) => acc + curr)->

Again, note this only has effect after saving (compiling so that the type parameter is instantiated). But it at least enables pipe completion in this scenario, even if it requires saving, which used to not work at all.

[cristianoc]

This seems to be a reasonable compromise.
I think the full solution would involve changing the case of application, which would have to find a way to instantiate function type parameters. That would need to get one's hands dirty with unification though, which is much more complex.

[zth]

@cristianoc ready for another look through, I think I fixed all of your points.

[cristianoc]

What about these changes?