Add a `check` submodule that provides a mini property-based testing framework

This module should be gated behind a check cargo feature.

It should provide a really simple property-based testing framework for combining Arbitrary-based generators with your oracles/property functions that can be used as smoke tests in local development and on CI, where it is often not always feasible to run the full cargo fuzz targets for a while due to various constraints (build time, length of time required to run, etc...). It would not be intended to replace long-running fuzzers leveraging coverage-guided feedback in the background, it would be a compliment to them for the local quick-test and CI uses previously described.

This is something I've wanted for a while, because I'm tired of doing it crappily by hand, but have obviously never gotten around to.

But, at some point, @matklad made the arbtest crate which is exactly what I'd been imagining, it's fantastic. Thank you very much for creating this, @matklad!

So my purposes for opening this issue are to gauge two things:

1. Other arbitrary maintainers: how do you feel about having such a feature/module in this crate?
2. @matklad: how do you feel about merging arbtest into this crate, as arbitrary::check?

The primary advantages I see of moving arbtest into arbitrary::check, instead of maintaining the current state with separate crates are:

• More discoverability
• Simpler usage for downstream users: don't need to depend on two different things; don't need to worry about version wrangling, especially if we ever make a breaking release of arbitrary
• Tighter integration: can make sure that existing arbitrary features are properly taken advantage of by arbtest/arbitrary::check and can make sure that as we develop new arbitrary features they are designed in such a way that arbtest/arbitrary::check can actually leverage them, resulting in better designs and features overall

So, what do y'all think?

Start by reviewing the proposed check cargo feature and the existing arbtest crate linked in the issue. Clarify maintainer and author agreement on whether the framework belongs in arbitrary, then define the integration scope and acceptance criteria before implementation.