Support for house numbers without street (addr:place schema)

This is a proposal to add support for addr:place by adding a new column 'addr_place' which works just like the 'street' column. A house number is simply interpreted as attachable to a street or a place (currently up to search rank 16) according to which of the two columns is not NULL. The search algorithm can remain unmodified. Requires [some modifications of osm2pgsql](https://github.com/lonvia/osm2pgsql/tree/place-housenumbers).

Works nicely with the Polish addr:place system, needs minimal modifications, can be adopted without reimport and has almost no overhead. Figures for planet import:

Import time: 2165 min (-43 min)
DB size: 524 GB ( +4 GB)

Downside is that I don't know how to make it work with the conscription number system without resorting to some ugly hacks. The whole double addressing idea probably requires to duplicate the housenumber column as well and possibly also parent_place_id, which in turn would have more severe implications for the front end.