This patch makes generating unique collections, such as "sets()" or
"lists()" with "unique=True", considerably more efficient (issue
#4458). Hypothesis no longer mutates test cases by copying one element
of a unique collection over another element of the same collection,
and no longer probes for minimal examples by drawing every element at
its simplest value - both of which were certain to be rejected, and
could waste several times as many draws from the element strategy as
were actually returned.
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