Flatten the crate's interface, to leak less internal structure

[sstangl]

This patch slightly modifies how items are exported so that they present a cleaner interface to a user.

The changes are slight. For example, they will see just KripkeStructure instead of kripke_structure::KripkeStructure, and they will see just futures::hilbert_epsilon instead of futures::hilbert_epsilon::hilbert_epsilon.

[rw]

@sstangl thanks!