269 improve shrinking prop flat map #513
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The tests didn't pass, and fixing them reliably seems impossible.
The previous shrinker always shrunk the inner strategy first. This seems to be the wrong thing to do in almost all non-trivial situations (e.g., generate a vector length and then a pair of vectors of this length). The new shrinker shrinks the outer strategy first (e.g., tries to minimize the vector length first and then minimizes the entries in the vectors). Given the current implementation of shrinking via simplify/complicate, I had a hard time doing this without cloning the inner strategy before simplifying it. So there's a breaking change here because prop_flat_map now requires the closure to produce a strategy that implements Clone. This could have been avoided by a change in how simplify/complicate operate, but this has the potential of breaking much more code than adding the requirement that the inner strategy is clonable.
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replaces #269. targets 2.x with breaking changes