BABE Epochs (#3028)

* Add `epoch` field to `SlotInfo`

* Add slot calculations

* More work on epochs in BABE

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <[email protected]>

* Typo: `/` not `%` for division

* Delete useless `LastSlotInEpoch::put(false)`

* Bump `spec_version`

* Make test suite pass again

* Implement BABE epoch randomness signing

* Try to fix compilation

Currently causes a stack overflow in the compiler

* Fix rustc stack overflow

* Add missing `PartialEq` and `Eq` implementations

* Fix compile errors in test suite

* Another silly compile error

* Clone `epoch`

* Fix compile error in benchmarks

* Implement `clone` for `Epoch`

* Merge master

* AUTHORING TEST PASSES!!!

* Fix compilation

* Bump `spec_version`

* Fix compilation

* Fix compilation (again)

* Remove an outdated FIXME

* Fix run.sh and move it to scripts/

* Delete commented-out code

* Fix documentation

Co-Authored-...