Use Style.t for ImageBackground.imageStyle (facebook#534)

Closes facebook#533

Author: MoOx

reason-react-native/src/components/ImageBackground.md

@@ -7,44 +7,13 @@ wip: true
 ```reason
 include NativeElement;
 
-type imageStyle;
-
-[@bs.obj]
-external imageStyle:
-  (
-    ~backfaceVisibility: [@bs.string] [ | `visible | `hidden]=?,
-    ~backgroundColor: Color.t=?,
-    ~borderColor: Color.t=?,
-    ~borderRadius: float=?,
-    ~borderBottomLeftRadius: float=?,
-    ~borderBottomRightRadius: float=?,
-    ~borderTopLeftRadius: float=?,
-    ~borderTopRightRadius: float=?,
-    ~borderWidth: float=?,
-    ~opacity: float=?,
-    ~overflow: [@bs.string] [ | `visible | `hidden]=?,
-    ~resizeMode: [@bs.string] [
-                   | `cover
-                   | `contain
-                   | `stretch
-                   | `repeat
-                   | `center
-                 ]
-                   =?,
-    ~overlayColor: Color.t=?,
-    ~tintColor: Color.t=?,
-    unit
-  ) =>
-  imageStyle =
-  "";
-
 [@react.component] [@bs.module "react-native"]
 external make:
   (
     ~ref: ref=?,
     // ImageBackground props
     ~imageRef: Image.ref=?,
-    ~imageStyle: imageStyle=?,
+    ~imageStyle: Style.t=?,
     // Image props
     ~accessibilityLabel: string=?,
     ~accessible: bool=?,

reason-react-native/src/components/ImageBackground.re

@@ -1,43 +1,12 @@
 include NativeElement;
 
-type imageStyle;
-
-[@bs.obj]
-external imageStyle:
-  (
-    ~backfaceVisibility: [@bs.string] [ | `visible | `hidden]=?,
-    ~backgroundColor: Color.t=?,
-    ~borderColor: Color.t=?,
-    ~borderRadius: float=?,
-    ~borderBottomLeftRadius: float=?,
-    ~borderBottomRightRadius: float=?,
-    ~borderTopLeftRadius: float=?,
-    ~borderTopRightRadius: float=?,
-    ~borderWidth: float=?,
-    ~opacity: float=?,
-    ~overflow: [@bs.string] [ | `visible | `hidden]=?,
-    ~resizeMode: [@bs.string] [
-                   | `cover
-                   | `contain
-                   | `stretch
-                   | `repeat
-                   | `center
-                 ]
-                   =?,
-    ~overlayColor: Color.t=?,
-    ~tintColor: Color.t=?,
-    unit
-  ) =>
-  imageStyle =
-  "";
-
 [@react.component] [@bs.module "react-native"]
 external make:
   (
     ~ref: ref=?,
     // ImageBackground props
     ~imageRef: Image.ref=?,
-    ~imageStyle: imageStyle=?,
+    ~imageStyle: Style.t=?,
     // Image props
     ~accessibilityLabel: string=?,
     ~accessible: bool=?,