Background

MPC now allows C#10.
C#10 introduced global using.

Problem

It causes name collisions.
To avoid those collisions mpc generated code must not use using at all.

Solution

Remove using directives from mpc templates.

Add new API byte[]? MessagePackReader.ReadByteArray()

MessagePackReader reader;
reader.ReadBytes()?.ToArray();

The current behaviour is the above.
It uses byte[] System.Buffers.BuffersExtensions.ToArray(ReadOnlySequence<byte>) extension method.
It cannot handle Nullable<ReadOnlySequence<byte>> well.