Expose a way to get the maximum encoded size for a give type

[jrmuizel]

This would be helpful speeding up serialization/deserialization because we can lift the length out completely and make it not dependent on the actual value.

[dtolnay]

Seems reasonable. I am closing the issue because I would prefer for this to live in a different crate -- it will need its own trait and derive macro to expose this information.

[jrmuizel]

I've started implementing this here: https://github.com/jrmuizel/bincode-size