Support for other reference systems than EPSG 3857

[pka]

Currently, only geometries in EPSG 3857 (Web mercator) are supported, despite an architecture which is targeted for generic reference system support. The main reason is a limitation of rust-postgis. It is planned to port the rust-postgis WKB reader (with support for SRID detection at runtime) to rust-geo, which solves other limitations as well.