Add Get Bytes Method Generator for C#

I updated idl_gen_general.cpp to add support for generating a Get Bytes
method for a vector to the generated C# source code. Given a byte vector
field named Foo, a method named GetFooBytes() will be generated in the
C# source code that will return an ArraySegment<byte> value referencing
the vector data in the underlying ByteBuffer.

I added a method to Table.cs named __vector_as_arraysegment that is used
by the code generated by the change to the C# generator.
__vector_as_arraysegment will take the offset of the vector and will
return the ArraySegment<byte> value corresponding to the bytes that
store the vector data.

I updated FlatBuffersExampleTests.cs to add tests to validate my
implementation of Table.__vector_as_arraysegment. I added tests to
demonstrate that the bytes for the monster's name can be extracted from
the underlying byte array. I also added tests to show that
Table.__vector_as_arraysegment returns a null value if the vector is not
present in the FlatBuffer.

I used the updated flatc.exe program to regenerate the C# source files
for the MyGame example. The new Monster class includes the GetXXXBytes
methods to return the byte arrays containing data for vectors.

net/FlatBuffers/Table.cs

@@ -67,6 +67,21 @@ namespace FlatBuffers
             return offset + bb.GetInt(offset) + sizeof(int);  // data starts after the length
         }
 
+        // Get the data of a vector whoses offset is stored at "offset" in this object as an
+        // ArraySegment<byte>. If the vector is not present in the ByteBuffer,
+        // then a null value will be returned.
+        protected ArraySegment<byte>? __vector_as_arraysegment(int offset) {
+            var o = this.__offset(offset);
+            if (0 == o)
+            {
+                return null;
+            }
+
+            var pos = this.__vector(o);
+            var len = this.__vector_len(o);
+            return new ArraySegment<byte>(this.bb.Data, pos, len);
+        }
+
         // Initialize any Table-derived type to point to the union at the given offset.
         protected TTable __union<TTable>(TTable t, int offset) where TTable : Table
         {