With this change, the domain of variable declarations can be specified.
Variables can have the integer domain, in which case additional integer-
specific simplification rules apply. Aside from that, the noninteger
domain represents precomputed values. An additional “unknown” domain is
introduced to flag variable domains prior to determining whether they
are integer or not.
This replaces the SimplificationResult enum class with OperationResult.
The rationale is that this type, which just reports whether or not an
operation actually changed the input data, is not simplification-
specific and will be used for integer variable detection as well.
The Tristate class (representing truth values that are either true,
false, or unknown) will be used at multiple ends. This moves it to a
separate header in order to reuse it properly.
Patrick Lühne