Implement support for custom shrinkers

[Seelengrab]

Currently, all shrinking is done naively & directly on the choice sequence (see here). This is neither customizable, nor anywhere close to optimal (and not at all how Hypothesis proper does it).

The plan for this is the following:

• Implement region tracking on a per-Possibility basis, preferrably automatically tracking which Possibility we're currently in
• Implement region shrinking, giving information about what the region represents
• Expose an interface for hooking into shrinking on a per-Possibility basis

My current idea is to follow what Hypothesis does about half way, informing how the regions are tracked. A custom shrinker would be given access to the part of the choice sequence it's responsible for, having it return a newly modified choice sequence as a replacement. It may be necessary to rework the interface surrounding Possibility a bit to do so - e.g. proptest returns a ValueTree when producing elements, similar to what PropCheck/Hedgehog do (except smarter when it comes to laziness of shrunk values).

[Seelengrab]

Idea: Require Possibility to implement the 2-arg produce, but only make it legal to call the 1-arg produce, even from within an existing produce. The reasoning is that the 1-arg produce can then attach every Possibility to a Shrinker, which can be potentially a custom one for that Possibility type. If the 2-arg produce is called directly, no shrinkers/metadata will be attached/managed.