make config::DefaultOptions::new const

[droundy]

We'd really appreciate having config::DefaultOptions::new() be const, so we can define our configuration as a static (or const) value.

Even better would be to implement const inherent methods duplicating the Options trait, so that an arbitrary configuration can be stored in a static value.

[VictorKoenders]

Heads up that bincode 1 is not actively being developed any more, so this will probably not be fixed before bincode 2 releases