tentative pipe syntax #2342

[bobzhang]

Note we only implement part of the proposal(#2342) in this PR:

x |. f --> f x 
x |. f a0 a1 --> f x a0 a1 
x |. f ~h:a0 a1 -> f x ~h:a0 a1 

Currently, the [@bs] will result in an unused attribute in such cases:

x |. f [@bs]
x |. f a0 a1 [@bs]

To keep things simple, we simply warn when in x |. f [@attrs], attirs is not empty, otherwise

• if we carry such attrs while not process it, (a warning produced but not idempotent)
• process again --> complicated not obvious benefit?

[bobzhang]

TODO: prohibit let (|.) a b = ... like what we did for ##+ , env.ml 1350