Allow function application on the right side of fast pipe.

Fast pipe desugars to |. Why? It's easy stub-able like: let (|.) = (a, f) => f(a);
This results into the rhs of -> not taking function application to make this work:
a->f(b) is parsed as (a->f)(b).
The fact that there isn't function application on the rhs, has resulted
into a wide range of problems with fastpipe.
In printing you actually want the layout to break as if
there was a function application on the rhs! The printing problems were
fixed at the cost of a tremendous amount of complexity in the printer.

Are the semantics (evaluation order etc) identical when defined in user space like this, as opposed to ppx?

No, they aren't…

```
let fn1 = (~foo=?, ()) => 1;
let fn2 = (~bar=?, x) => 2;

/* Ok */
fn1(~foo=1, ())->(fn2(~bar=2))

/* Ok */
fn1(~foo=1,()) |. fn2(~bar=2)

/* Error: This expression has type int. It is not a function. */
fn1(~foo=1, ())->fn2(~bar=2)

/* Explanation */
fn1(~foo=1, ())->fn2(~bar=2)

/* this translates to: */
fn2(1)(~bar=2)

/* however, because `~bar` is optional, `fn2(1)` already produced a value. Therefore: */
2(~bar=2)
```

In retrospect, not having function application on the rhs was a terrible mistake.
Fast pipe is all about function application in its current form.
If it was done via ppx, then you wouldn't get the error in the example
above.

Author: IwanKaramazow

esy.json

@@ -32,6 +32,7 @@
     "install": [
       ["esy-installer", "reason.install"],
       ["esy-installer", "rtop.install"]
-    ]
+    ],
+    "buildsInSource": "_build"
   }
 }

formatTest/errorTests/expected_output/reservedField.re

@@ -1,2 +1,2 @@
 File "reservedField.re", line 1, characters 11-15:
-Error: 1050: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecord.re

@@ -1,2 +1,2 @@
 File "reservedRecord.re", line 1, characters 24-28:
-Error: 1821: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordPunned.re

@@ -1,2 +1,2 @@
 File "reservedRecordPunned.re", line 1, characters 22-26:
-Error: 3369: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordType.re

@@ -1,2 +1,2 @@
 File "reservedRecordType.re", line 1, characters 11-15:
-Error: 766: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead

formatTest/errorTests/expected_output/reservedRecordTypePunned.re

@@ -1,2 +1,2 @@
 File "reservedRecordTypePunned.re", line 1, characters 11-15:
-Error: 766: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead
+Error: type is a reserved keyword, it cannot be used as an identifier. Try `type_` or `_type` instead