Add support for evaluation of type-level natural numbers.
This patch implements some simple evaluation of type-level expressions featuring natural numbers. We can evaluate *concrete* expressions that use the built-in type families (+), (*), (^), and (<=?), declared in GHC.TypeLits. We can also do some type inference involving these functions. For example, if we encounter a constraint such as `(2 + x) ~ 5` we can infer that `x` must be 3. Note, however, this is used only to resolve unification variables (i.e., as a form of a constraint improvement) and not to generate new facts. This is similar to how functional dependencies work in GHC. The patch adds a new form of coercion, `AxiomRuleCo`, which makes use of a new form of axiom called `CoAxiomRule`. This is the form of evidence generate when we solve a constraint, such as `(1 + 2) ~ 3`. The patch also adds support for built-in type-families, by adding a new form of TyCon rhs: `BuiltInSynFamTyCon`. such built-in type-family constructors contain a record with functions that are used by the constraint solver to simplify and improve constraints involving the built-in function (see `TcInteract`). The record in defined in `FamInst`. The type constructors and rules for evaluating the type-level functions are in a new module called `TcTypeNats`.
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