Add support for single line comments of the form

Author: IwanKaramazow

esy.json

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

formatTest/unit_tests/expected_output/basicStructures.re

@@ -146,7 +146,7 @@ let x = arr^[0] = 1;
  *============================================================================
  */;
 
-let (/\/) = (+); /* // is not a comment */
+let (/++) = (+); /* // is not a comment */
 
 let something =
   if (self.ext.logSuccess) {

formatTest/unit_tests/expected_output/emptyFileComment.re

@@ -0,0 +1 @@
+// file with just a single line comment