In preparation for some upcoming work on the shrinker, this PR changes
the interfaces of Fuzzer, Replayer and Shrinker so that all data
relevant to each class is provided on construction, meaning that the
"Run" method can become a zero-argument method that returns a status,
transformed binary and sequence of applied transformations via a
struct.
This makes greater use of fields, so that -- especially in Fuzzer --
there is a lot less parameter passing.
This change adds the notion of "overflow ids", which can be used
during shrinking to facilitate applying transformations that would
otherwise have become inapplicable due to earlier transformations
being removed.