PmCheck: Only ever check constantly many models against a single pattern

Introduces a new flag -fmax-pmcheck-deltas to achieve that. Deprecates the old -fmax-pmcheck-iter mechanism in favor of this new flag.

From the user's guide:

Pattern match checking can be exponential in some cases. This limit makes sure we scale polynomially in the number of patterns, by forgetting refined information gained from a partially successful match. For example, when matching x against Just 4, we split each incoming matching model into two sub-models: One where x is not Nothing and one where x is Just y but y is not 4. When the number of incoming models exceeds the limit, we continue checking the next clause with the original, unrefined model.