Write spec for message execution

[whyrusleeping]

Description

One of the many pieces missing from the current spec is a description of how messages are evaluated, and how they modify the state. We should add this in to the 'state-machine.md' doc.

Acceptance criteria

I don't think its necessary to talk about the details of the go code running the contracts at this point, but just talking about what the inputs and outputs of that bit are, and then maybe a pointer to where the 'canonical' code lives, for now.

What I would like, is the information necessary to take a state tree, and apply a message to it.

Risks + pitfalls

Where to begin

[whyrusleeping]

I think in a future doc, once we're closer to being 'complete', we can write up in detail (or in some language) exactly how each of the builtin actors methods works.

Also, I think that @acruikshank would be a good candidate for writing this one.

[mishmosh]

New plan is for @phritz or @acruikshank to braindump to someone who is interested in learning this and can draft the spec. Looking for a taker :)