Made JsValue's layout explicit

Because a struct is either a bool or a double or a string
pointer...etc., the layout can be optimized by setting all the fields to
the same location and assigning the important field last, which makes
its memory be notable. This makes the struct a guaranteed 96 bits,
rather than 232.

Jint/Native/JsValue.cs

@@ -15,9 +15,11 @@ using Jint.Native.String;
 using Jint.Runtime;
 using Jint.Runtime.Interop;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace Jint.Native
 {
+    [StructLayout(LayoutKind.Explicit)]
     [DebuggerTypeProxy(typeof(JsValueDebugView))]
     public struct JsValue : IEquatable<JsValue>
     {
@@ -28,20 +30,22 @@ namespace Jint.Native
 
         public JsValue(bool value)
         {
-            _bool = value;
             _double = double.NaN;
             _object = null;
             _string = null;
             _type = Types.Boolean;
+
+            _bool = value; //Set value last because of 'FieldOffset' constraints
         }
 
         public JsValue(double value)
         {
             _bool = false;
-            _double = value;
             _object = null;
             _string = null;
             _type = Types.Number;
+
+            _double = value;
         }
 
         public JsValue(string value)
@@ -49,17 +53,19 @@ namespace Jint.Native
             _bool = false;
             _double = double.NaN;
             _object = null;
-            _string = value;
             _type = Types.String;
+
+            _string = value;
         }
 
         public JsValue(ObjectInstance value)
         {
             _bool = false;
             _double = double.NaN;
-            _object = value;
             _string = null;
             _type = Types.Object;
+
+            _object = value;
         }
 
         private JsValue(Types type)
@@ -71,12 +77,16 @@ namespace Jint.Native
             _type = type;
         }
 
+        [FieldOffset(0)]
         private readonly bool _bool;
 
+        [FieldOffset(0)]
         private readonly double _double;
 
+        [FieldOffset(0)]
         private readonly ObjectInstance _object;
 
+        [FieldOffset(0)]
         private readonly string _string;
 
         private readonly Types _type;