mvp commit validation

[ebuchman]

First take at verifying a commit using the existing vote and validator structures.

Note the commit structure in Tendermint is scheduled to change: tendermint/tendermint#1648

It might be better to have a version of this function that is generic in the vote type.

[ebuchman]

This is mostly replaced by #31 but we'll probably want some of those to_amino methods for the actual implementations so we can get the sign bytes right. cc @liamsi

[liamsi]

Yes, I can probably re-use some of the to_amino methods. It might be slightly more idiomatic to implement the From trait instead to convert to the amino types though:
https://doc.rust-lang.org/std/convert/trait.From.html

A slightly more annoying usage of amino occurs when creating the header hash as we need to encode field by field.

[ebuchman]

Replaced by #31 and #36