Fix type of Nullable.t which was not untagged in the implementation.

Author: cristianoc

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Next version
 
 - BREAKING: Fixes the type of `RegExp.Result.t` to be `array<option<string>>` instead of `array<string>`. https://github.com/rescript-association/rescript-core/pull/234.
+- Fix type of `Nullable.t` which was not untagged in the implementation. https://github.com/rescript-association/rescript-core/pull/235
 
 ## 1.4.0
 

src/Core__Nullable.res

@@ -1,3 +1,4 @@
+@unboxed
 type t<'a> = Js.Nullable.t<'a> =
   | Value('a)
   | @as(null) Null