Builds an AST visitor based on the default implementation, merged with the one you provide.

Example of visitor which counts references to a column 'foo':

let cnt = 0;
const visitor = astVisitor(v => ({
     ref: r => {
         if (r.name === 'foo') {
             cnt++;
         }
         v.super().ref(r);
     }
 }));

visitor.statement(myStatementToCount);
console.log(`${cnt} references to foo !`);