Introduce traits for collection containers

[frankmcsherry]

This PR introduces traits Negate, Enter, Leave, and ResultsIn for containers, such that collections based around containers implementing each of these traits gets access to the corresponding methods.

[frankmcsherry]

In principle this should be extendable to the Iterate trait, but .. it's a bit weird in that the trait now needs to name two container types, the "returned" type and the "iterated" type. All possible, but a bit wordy and perhaps the next steps should be to adapt examples/columnar.rs to use variables first, to ensure that the traits check out.

[antiguru]

Looks great!