This switches out the `<variant/optional.hpp>` implementation of the
optional monad to the one from Boost.
The following trick makes sure we keep compile times down:
- use `<boost/optional/optional_fwd.hpp>` to forward declare the
optional type in header, then include the full blown optional header
only in the implementation file.
- do the same for the files we touch, e.g. forward declare osmium types,
allowing us to remove the osmium header dependency from our headers:
`namespace osmium { class Relation; }
and then include the appropriate osmium headers in the implementation
file only. We should do this globally...
References:
- http://www.boost.org/doc/libs/1_59_0/libs/optional/doc/html/index.html
- https://github.com/osmcode/libosmium/issues/123
