object method call simplification

[bobzhang]

Thanks to the recent clean up, there is no need to apply a ppx for method call.

obj["say"] was currently compiled into obj##say, and transformed again in the compiler into Pexp_send with some wrappers.

This is not needed any more, it can be translated into obj#say without any post-processing.

To finish this, we should also implement the printer logic for Pexp_send

[chenglou]

What happens to #=?

Edit: seems mixing the two causes an assertion error:

[bobzhang]

This is not finished yet. currently you use person##name #= "hi", but we will support person#name #= "hi" soon.
Note I tested the assignment in rescript syntax, it does not seem correct.

[bobzhang]

@chenglou The error is fixed in rescript-lang/rescript#4986. So we can simplify the method call in ReScript syntax now.