irmin.type: pp_ty should show components of algebraic types

[craigfe]

Currently, Irmin.Type.pp_ty pretty-prints any variant/record generics as "Variant"/"Record" respectively (see https://github.com/mirage/irmin/blob/master/src/irmin/type/type_pp.ml#L56-L57), which is not particularly helpful. It could contain:

• the name of the variant/record;
• the pretty-printed form of the component types.