Follow-up to #5419

The C# ByteBuffer class features a number of generic methods which are employed mostly when dealing with vectors. The generic methods work using a static table of type sizes, and do not support any type not in the table, including the enum types generated from flatbuffer schemas. They also use the Buffer.BlockCopy method to copy directly between the backing byte array and strongly typed arrays returned to or supplied from the user, and this method does not work with enum-typed arrays.

The C# code generator works around ByteBuffer's lack of support for enum types by casting to/from the generated enum types when using the various Get*/Put* methods for reading/writing individual integer values. But for some generated code this work-around is not applicable, and flatc generates code which invokes the generic ByteBuffer methods using enum types, which will always throw at runtime.

I considered first attempting to address the limitations in ByteBuffer instead, but saw no way to do this without negatively impacting either performance or compatibility.

My solution modifies the generator to avoid calling the generic methods with enum types:

• Generated static Create*VectorBlock() methods now delegate to their Create*Vector() counterparts, which are semantically identical but slower. They should really be removed but I kept them there for backwards compatibility.
• Generated Get*Array() methods now directly create an array and copy data into it in a loop instead of calling __vector_as_array<T>(). Data is read into the array the same way the standard accessors for enum vectors read from the buffer, by using a specific Get*() method and casting to the enum type. The loop is roughly modelled after similar loops generated in Create*Vector() methods.