Support function "abstrjson" (GF newer than v3.3.3)

[Kaljurand]

In abstrjson

Pred (That Fish) Fresh

is represented as

{fun:"Pred",fid:3,
 children:[{fun:"That",fid:1,
        children:[{fun:"Fish",fid:0}]},
       {fun:"Fresh",fid:2}]}