New statement "map_to_area" for area in area queries

"Map_to_area" statement's objective is to convert way/relation ids to their corresponding area ids. It can be used for area in area queries, which are not possible as of now (see issue #45). In a way, it works like the pivot filter, just in the opposite direction.

Coding was testing using the following example on a **local Overpass API instance**

http://overpass-turbo.eu/s/56K 

Code may be slow and very likely needs some further polishing. Attic part was not tested due to missing test data on local instance.

Feedback welcome!