Move key types into World module

Move Left_Or_Right, Sense_Dir, Condition, InsState and Instruction so
that they can be used by modules that import World. Added mkMarker to
construct Markers from Integers.
Merge branch 'master' of https://github.com/londonhaskell/feeding-haskell-to-ambiants
Added AntCode module for writing Ant brains

Implementation based on a Free Monad and explicit jumps for state
machine changes that are not just to the immediately following command
Merge pull request #13 from derekwright/master

Add AntCode.hs