chore(code): Split HostMsg out of engine crate and into a new host crate #1106
Description
Now that https://github.com/informalsystems/malachite/pull/1109 is merged, we should be able to split the HostMsg enum out of the malachitebft-engine crate and into its own malachitebft-host crate. We can then re-export it from the malachitebft-engine crate to avoid introducing a breaking change.