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.)
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