Inefficient `$regex` Mango `view` queries on indexed fields

On the Mango query interface, when the `$regex` operator is used in a selector with a field that could be backed up by a `view` (`json`) index, it is not leveraged. The use of `$regex` in its current form rather implies a brute-force scan through all the documents of the underlying view, which makes the whole approach inefficient and resource-intensive.

The source of the problem is that no index ranges could be computed for `$regex` (to narrow down the scan) because regular expressions do not naturally generate an ordering on the set of strings, like for example comparisons do on integers. However, there is a prior art in for example, Lucene where it is possible to achieve that for specialized cases, such as "starts with"-type of expressions, i.e. `^foo.*`.

Some more links about how Lucene does it, just for inspiration:
- https://lucene.apache.org/core/4_2_0/core/org/apache/lucene/search/RegexpQuery.html
- https://lucene.apache.org/core/4_2_0/core/org/apache/lucene/search/AutomatonQuery.html
- https://blog.mikemccandless.com/2011/03/lucenes-fuzzyquery-is-100-times-faster.html
- https://www.slideshare.net/otisg/finite-state-queries-in-lucene

Another factor that can slow down the evaluation is that documents are included unconditionally, and `$regex` is not matched against the key itself first and only documents for the matched keys are fetched and returned. Something along the lines of [covering indexes](https://github.com/apache/couchdb/issues/4413).

Start with the Mango query interface and the behavior of `$regex` against `view` (`json`) indexes. Review the Lucene examples and the linked covering-index issue, then define and test a bounded optimization for suitable prefix expressions while avoiding unconditional inclusion of unrelated documents.

Written by the indexing model from the issue text.