• Adds a new combinator, Gen.weighted. It's like frequency but doesn't require so many Gen.pure calls.
• The constraints on Gen.frequency and Gen.weighted are now relaxed to allow any SequenceType
• The shrinker loop is thinner, quicker, and much much less memory hungry than before for complex shrinks.
• Fixes a bug in the shrinker where tests would not report the number of shrinks they'd gone through.
• Propagates expected failures and outputs the correct Result type for them.
• Compile times have been significantly improved for some files (h/t @robrix).