support type alias statements in surface syntax

[zzlk]

If we're going to support use statements then type statements are probably necessary as well.

[zzlk]

For example:

    let mut df = hydroflow_syntax! {
        use lattices::map_union::{MapUnionHashMap, MapUnionSingletonMap};
        type VecClock = MapUnionHashMap<usize, Max<usize>>;
    }

[jhellerstein]

to be fixed by Hydroflow+