Introduce boxed methods for insertion.

[CoolSlimbo]

PR introduces additional methods that allow using boxed versions of the traits directly.
Previous methods have been adapted to use the boxed versions underlying, and the boxed methods run all the (previous) logic to them.

Relevant issue: #620

[alice-i-cecile]

Once you add a note to RELEASE.md I'll merge this in :) Thanks!

I think this is a bit clearer / less messy than impl Trait for Box<dyn Trait>.

[CoolSlimbo]

Finalizing release notes now (suspecting this would be under 0.16.1 as a minor push?), was thinking, should this be listed in the readme as a feature? Especially since it's prime use case is allowing serialized types to be imputed?

[alice-i-cecile]

Once we have a full test / example for serialization, yes please to putting it in the README. Until then, we shouldn't make promises that aren't tested :) Please feel free to add anything else required to fix #620: this is a feature I actively want.