Added support for C# partial class generation when the `csharp_partial` attribute is used on a table/struct.

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.

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.

```
namespace MyGame;

enum CommandType : byte {
	None = 0,
}

table Command {
	id:int;
	type:CommandType;
}
```

then generate c# files. it'll output compile error like these.

```
Assets/MyGame/Command.cs(18,39): error CS1041: Identifier expected
Assets/MyGame/Command.cs(18,39): error CS1737: Optional parameter cannot precede required parameters

16:   public static Offset<Command> CreateCommand(FlatBufferBuilder builder,
17:   int id = 0,
18:   CommandType type = (CommandType)0) {
```

Change-Id: I3cdd801b641bf180222203a37634598350d340e3

When creating a “CreateXxx(...)” method for a “simple table” type,
enum-type fields with a non-zero default must have an explicit
cast for the respective argument default value, because in C#,
there is an implicit cast from int to an enum only for 0.

Also, added an example of such type into the example monster_test
type, so that we test this feature.