Fix C# vector of enum code generation

Fixes a bug where the logic to determine when to use a C# enum flags
both enums and vectors of enums.  This causes the C# generator to
generate code that doesn't compile for tables that contain vectors of
enums.

The fix also consolidates type generation functions a bit and adds
some additional casting functions for clarity.

tests/MyGame/Example/Vec3.cs

@@ -29,7 +29,7 @@ public sealed class Vec3 : Struct {
     builder.PutSbyte(test3_B);
     builder.PutShort(test3_A);
     builder.Pad(1);
-    builder.PutSbyte((sbyte)(Test2));
+    builder.PutSbyte((sbyte)Test2);
     builder.PutDouble(Test1);
     builder.Pad(4);
     builder.PutFloat(Z);