need a new function to extract selected object pairs

In order to assist a couple of customers who are doing time series work, we need a new function that given an object will produce an array of object pairs selecting only fields which match a certain pattern.
The idea here is since scanning is much faster than unmarshalling, we can unmarshall only those fields that qualify and we can further optimize things by producing an array whose "val" elements are themselves delayed parsing documents.
In tests done for MB-30695 we can lower latency of an unmarshall intensive query by 25% just by using this trick.
While we are doing the work - we can tighten the code for object_names(), object_pairs() and object_values().