function compile(schema, _meta) {
  var schemaObj = this._addSchema(schema, undefined, _meta);
  return schemaObj.validate || this._compile(schemaObj);
}
Create validating function for passed schema.
@this   Ajv
@param  {Object} schema schema object
@param  {Boolean} _meta true if schema is a meta-schema. Used internally to compile meta schemas of custom keywords.
@return {Function} validating function