This merges the four maps into a single map, each entry of which tracks
the progress of both the `Broadcast` and the `BinaryAgreement`
subalgorithm for an individual proposer. Two advantages:
* A slight optimization, because the `Broadcast` instance is dropped as
soon as it is not needed anymore.
* The static guarantee that certain impossible situations (inserting a
broadcast value twice) cannot happen.
The module is also split up into smaller files.
Vladimir Komendantskiy
Andreas Fackler
Demi Marie Obenour
Andreas Fackler