feat: MemoryPackSerializer.ResetState, ResetReaderState, ResetWriterState

[alexyakunin]

Sometimes you want to invoke MemoryPackSerializer.Serialize while serializing another type (e.g. to serialize a complex nested structure into a byte array on serialization). And sometimes you want to do the same on deserialization. Since MemoryPack uses [ThreadStatic] to store its serializer/deserializer state, this doesn't work - unless you backup its state & reset it.

The provided implementation does exactly this. Currently we have to resort to CreateDelegate/ILEmit-based workaround to address that, this change would address the problem in more straightforward way.

[neuecc]

Thanks, it seems like a good approach.
Could you please add unit tests and sample code to the README?

[alexyakunin]

Will do somewhere on this week. Thanks!