This PR improves support for `Nat` in the `cutsat` procedure used in
`grind`:
- `cutsat` no longer *pollutes* the local context with facts of the form
`-1 * NatCast.natCast x <= 0` for each `x : Nat`. These facts are now
stored internally in the `cutsat` state.
- A single context is now used for all `Nat` terms.
The PR also introduces a mapping mechanism for all "foreign" types that
can be converted to `Int`. Currently, only `Nat` is supported, but
additional types will be added in the future.
3b78ada5d8