Add new microblock type

[jbencin]

As I understand, due a recent PR we now have two different types of microblocks: The regular ones with transactions, and ones that contain "rollups" of previous microblocks. It is somewhat confusing to have a single type that has different meanings depending on context, so we should make this "microblock rollup" a different type of block, even if it's just renaming the struct in Rust.