Cleanup pipe: remove |.u and change "|." to "->" to represent pipe.

[cristianoc]

AST cleanup: remove "|." and rename "|." to "->" in the internal representation for the pipe operator.

[cristianoc]

@cknitt @zth any idea if someone in a ppx might be relying on pipe being represented as "|."? Seems unlikely but just double checking.
As in that case, we might have to convert back and forth when calling a ppx. Which would be a time bomb anyway, as the problem would be pushed to when the new ast is adopted to ppxs.

[zth]

I think PPXes do rely on that, yes. So doing a transform would be good.

I envision all of the major PPXes in the ecosystem moving to the proper ReScript AST once we've cleaned it up and are ready for consumption.

[zth]

Looks great! Just the question about the PPX transform.