fix(content): FIL VM State Tree Update

[yiannisbot]

This PR is an update to the VM state tree and the (most important) functions related to updating it.

@anorth please advise on important things not currently mentioned, as well as on whether the TODO at the end is still relevant or not.

[schomatis]

I'm not sure if we want to make the snapshot system part of the spec, other than that I have little to add here.

[yiannisbot]

I've included mention of the snapshot system to hint on the fact that a view of the current state is needed/is useful in multiple occasions. I believe this is useful for any implementation to have, but I'm not strong about it. I can either enhance the text to make the point clearer if we want to have it in, or exclude if not.

[anorth]

This all just looks like Lotus implementation detail to me. The only piece that is relevant to other implementers is that the top layer of the state tree is a HAMT.

[yiannisbot]

@anorth I've excluded implementation details on snapshots as well as the code snippets and added a link to the lotus repo.

[yiannisbot]

Thanks @anorth! Fixed some links to external modules and set status to "reliable".

@hugomrdias let's merge this.