PushInto targets container instead of item

[antiguru]

Update to flatcontainer 0.3.0, which changes the CopyInto trait into a Push trait. Update Timely's PushInto trait in the same manner, i.e., the parameter is what should be pushed and self is the container.

Split PushContainer into a push function on Container and a SizableContainer that knows capacity, preferred_capacity, and resize functions, which the capacity container builder and PushPartitioned require. It also appears as a restriction on ContainerBuilder, as did PushContainer in the past.

Adds a implementation for flatcontainer::Region for TimelyStack, it's not used yet but would allow us to store columnated data in flatcontainer.

[frankmcsherry]

Looks good. A few questions, but nothing deeply consequential.