Implement OVERLAPPING and OVERLAPPABLE pragmas (see #9242)
This also removes the short-lived NO_OVERLAP pragama, and renames OVERLAP to OVERLAPS. An instance may be annotated with one of 4 pragams, to control its interaction with other overlapping instances: * OVERLAPPABLE: this instance is ignored if a more specific candidate exists * OVERLAPPING: this instance is preferred over more general candidates * OVERLAPS: both OVERLAPPING and OVERLAPPABLE (i.e., the previous GHC behavior). When compiling with -XOverlappingInstances, all instance are OVERLAPS. * INCOHERENT: same as before (see manual for details). When compiling with -XIncoherentInstances, all instances are INCOHERENT.
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