Allow writing at arbitrary positions in MessagePack::Buffer about msgpack-ruby

[frsyuki]

Sounds good and reasonable idea.
My understanding is to add Buffer#move_to(buffer) method which moves internal data from the receiver to the argument without copying the data.

[chrisseaton]

I'm suggesting we close this because:

• it's speculative feature request - can't be too critical to have as it's not been mentioned since 2013
• MessagePack::Buffer is not supposed to be a general utility

If someone can show a pressing requirement, or a good example of where it makes code using this gem much better, we can reconsider.