Accessor API for report returned by @check

[Seelengrab]

Currently, the report returned by @check is an opaque object with lots of internals. This can be quite annoying when there is some programmatic manipulation or inspection required, such as

• when a NaN with a non-standard bitpattern causes a property to fail (it's all printed as NaN, so that hides the failure)
• when a user wants to build on top of @check without going through the entire AbstractTestset machinery for wrapping.
• when hypothetical statistics about the generation & shrinking process are interesting of themselves, for example for a visualization

For cases like these, it would be good to have an official API for retrieving that information from a report.

[aplavin]

First and foremost, boolean ispass would be nice :)

when a NaN with a non-standard bitpattern causes a property to fail (it's all printed as NaN, so that hides the failure)

All kinds of types have custom printing, so extracting the failed counterexample is useful more generally than for NaNs.