Deprecate the max_shrinks setting

[Zac-HD]

As discussed in #535, the max_shrinks setting is a mess. There's no way to pick a meaningful value for it beyond "none, default, lots". For "none" it's actually better to disable shrinking using the phases setting, because max_shrinks=0 behaves identically to max_shrinks=1.

The setting should be removed; max_shrinks=0 translated to phases=tuple(Phase)[:Phase.shrink] (deliberately changing behaviour to match expectation).

It would be nice to have an improved heuristic for shrinking - aiming to fully shrink large examples, but bail out of pathological cases quickly - but for the first pass we can simply hard-code the current default behaviour. A second pass should consider #233.

master...Zac-HD:no-max-shrinks is an updated copy of the work I did in #1211 before giving up; the tests almost all pass but there are a few that depend on the one shrink/no shrinks distinction and can't afford the perf cost of shrinking.