Consider improvements to the `teardown` syntax.

[obmarg]

If a fixture needs to do some teardown, at the moment it ends up looking a bit like this:

deffixture db do
   db = setup_db
   teardown fn ->
     close_db(db)
   end
   db
end

It might be nice to provide a better mechanism for teardown code. py.test has it's yield fixtures which are pretty nice, but (failing anything nicer) we could maybe provide a macro named yield/return/something that provides a bit of sugar around the above:

deffixture db do
   db = setup_db
   yield db, teardown: fn ->
     close_db(db)
   end
end