Don't recreate example groups for mutation except when necessary

If data hasn't changed since the last iteration of the mutator loop,
then the example groups can be reused, and don't need to be
recalculated.

This restores a noticeable chunk of the performance lost in HypothesisWorks#2308.

hypothesis-python/src/hypothesis/internal/conjecture/engine.py

@@ -687,6 +687,7 @@ def should_generate_more():
             ):
                 initial_calls = self.call_count
                 failed_mutations = 0
+                groups = None
                 while (
                     should_generate_more()
                     # We implement fairly conservative checks for how long we
@@ -695,11 +696,12 @@ def should_generate_more():
                     and self.call_count <= initial_calls + 5
                     and failed_mutations <= 5
                 ):
-                    groups = defaultdict(list)
-                    for ex in data.examples:
-                        groups[ex.label, ex.depth].append(ex)
+                    if groups is None:
+                        groups = defaultdict(list)
+                        for ex in data.examples:
+                            groups[ex.label, ex.depth].append(ex)
 
-                    groups = [v for v in groups.values() if len(v) > 1]
+                        groups = [v for v in groups.values() if len(v) > 1]
 
                     if not groups:
                         break
@@ -750,6 +752,7 @@ def should_generate_more():
                         )
                     ):
                         data = new_data
+                        groups = None
                         failed_mutations = 0
                     else:
                         failed_mutations += 1