Assign empty labels to array elements for better efficiency

[UnkindPartition]

Right now this package assigns consecutive integer numbers (serialized as strings) for array labels. This is wasteful in both space and time.

This PR addresses the issue as far as possible within the BSON spec.

In my example, this reduced the serialziation time from 1.5s to 1s and the file size from 45Mb to 32Mb.

[VictorDenisov]

Hi @feuerbach can possibly see a reason for using consequtive integers array labels?

[VictorDenisov]

It looks like bson spec explicitly suggests to use consequtive ascending order of labels: http://bsonspec.org/spec.html (look for notes). Can we optimize it somehow without dropping consequtive labels?

[UnkindPartition]

I see. Well, you could replace (T.pack $! show i) with a call to a more efficient serialization function that goes from Int to Text directly — there must exist some packages for that.

Also, a comment in the code citing/linking to that part of the spec would be helpful. Thanks.