We use markers to mark a direction home and, when one is known, a
direction to food.

The mapping is as follows:

Direction              Marker
                   2     1     0
----------------------------------
0 (right)      |  set  unset  set
1 (down right) |  set   set  unset
2 (down left)  | unset  set   set
3 (left)       | unset  set  unset
4 (up left)    | unset unset  set
5 (up right)   |  set  unset unset

(with food markers using 5,4,3 rather than 2,1,0)

Note that this means that during the startup phase (startup.txt) all of
the ants are laying down arrows such that they point into the keeper
structure. Whenever we move into a hex without a home marker we leave
one pointing the way we came.

As an optimisation, if we want to say direction 3, say, rather than
read 0, read 1, read 2, write 1
we can do
read 0, read 2, write 1
as rewriting mark 1 won't matter.