Refactored the Java and C# code generators into one.

Also made the C# implementation support unsigned types, and
made it more like the Java version.

Bug: 17359988
Change-Id: If5305c08cd5c97f35426639516ce05e53bbec36c
Tested: on Linux and Windows.

tests/FlatBuffers.Test/Assert.cs

@@ -63,7 +63,7 @@ namespace FlatBuffers.Test
             {
                 action();
             }
-            catch (T ex)
+            catch (T)
             {
                 caught = true;
             }