0.21

This release offers better negative test integration and furthermore fixes a couple of bugs in QCheck.Shrink and in ppx_deriving_qcheck:

• make Test.check_result, Test.check_cell_exn, and Test.check_exn honor test polarity by raising
  Test_unexpected_success when a negative test (expected to have a counter example), unexpectedly succeeds.
• fix issue with ppx_deriving_qcheck deriving a generator with unbound gen for recursive types #269
  and a related issue when deriving a generator for a record type
• fix #241 causing QCheck.Shrink.int* to emit duplicates, also affecting QCheck.Shrink.{char,string}
• fix a cornercase where Shrink.list_spine would emit duplicates