Type alias for block type

[jbearer]

Throughout our repos, we use u64 for block height. It would improve readability to have a standardized type alias (or maybe even a newtype) BlockHeight. I think we don't actually use height much in HotShot, more downstream, but it is a field of the leaf, so I think defining this type alias here makes sense, and will allow all downstream repos to import it so we have a consistent use of types across repos.