Logic::Puppet implements an Interpreter::Puppet.
Add measurements.hpp for step and tick types.

This way everybody can use them without including stuff they don't
otherwise need.
Interpreter now a member of World.

It's better design this way, except for the part about storing the
subprograms until the Interpreter is actually created. That, and
the handling of the Server::Clients is atrocious.
Client no longer holds a reference to Summoner.

Functionality that worked yesterday works again.
Fix coord's comparison operator.

Wasn't strict weak ordering, and I'm stupid.
Refactored Server::Client to be a bit less ugly.

It's still not nice, mind you. I always have trouble with these
simple descriptor structs that end up needing to be full-blown
classes.
Puppets are turning, and send messages about it.
Server sends messages about summoning.

Two important things:
 * the message is put in the queue and sent only on tick, well after
   the reply.
 * due to this, World::outbox is emptied by the server.
Dunno if these are good.
Puppets actually move.

They bump into "walls" when moving out of map, and they happily bump
into each other.