Functions to manipulate candidates during the solution search now
always return a Result instead of sometimes returning a bool, and
other times returning an Option.
We opted to use Result over Option mainly to (ab)use the try! macro.
In addition, it also makes it emit a warning when we forget to handle
the result. (We tend to assume people don't just ignore the result
and continue as normal, instead of putting the puzzle in a special
contradiction detected state.)
Constraints must now implement the substitution of "from" with "to".
The implementations can be sanity checked when we come to build the
list of constraints that each variable wakes up.
In the future, we would like to make the more general substitution:
from -> scale * to + constant.
