Implement Basic TipSet Manager about forest

[ansermino]

fn ticket_from_round // returns the ticket that is at round 'r' in the chain behind 'head'

seems we need some notion of round. Any thoughts on whats needed for this?

fn add_losing_ticket // adds the losing ticket to the chaintips manager so that blocks can be mined on top of it

why loosing ticket?

[dutterbutter]

Based on the specification, it seems they are using the term round interchangeably with epoch. They define an epoch as:

An epoch refers to the period over which a given random seed is used in Expected Consensus.
In the current Filecoin implementation, each round is an epoch.

@austinabell has stubbed ChainEpoch in ferret/vm/src/runtime/mod.rs but looks like it still needs to be formally defined.

As for the losing ticket I am not sure why they are referring it to as losing...based on everything else I've read it should be the winning ticket that blocks are mined on top of BUT the ChainTipsManager in the spec indicates losing...

[dutterbutter]

Closing this issue as I believe this to be essentially the TipIndex tracker, just the wording chosen in the spec is poor. For example, tipset_from_round is essentially get_tipset_by_epoch which is included in #113