Teach compiler that fold and map with clean procedure is itself clean

[sjamaan]

It looks like CHICKEN currently doesn't understand that fold and map are #:clean if the argument procedure is #:clean. This prevents it from recognising that something like (fold + 0 (iota 10000)) is foldable.

We could hardcode these procedures, but I'd prefer not to, so this might require some kind of new annotation in types.db.

This idea was inspired by ​http://pramode.in/2016/08/25/rust-add-billion-numbers/ - if Rust can do it, so can we!