2 0 for real

[Dr-Emann]

Add some tests which will fail until we get a 4.0.1 of CRoaring. Don't merge until we update again once CRoaring is fixed.

Also, include the matching CRoaring version in the readme and some clarification on versioning, this will hopefully help clarify somewhat the questions raised in #136. Link to the rendered added section: https://github.com/Dr-Emann/croaring-rs/blob/2_0_for_real/README.md#croaring-version

Fixes #136

[Dr-Emann]

@saulius, what are your thoughts on matching the CRoaring versioning exactly (bumping croaring-sys to 4.0.0) for croaring-sys, at least the major versions could probably match, any major version bump for them is almost guaranteed to be a major version bump for it, and I don't foresee any reason we'd make breaking changes to the sys crate other than updating to a new CRoaring version.

[saulius]

@Dr-Emann I'd agree for the croaring-sys to match the version. For the croaring crate I'd say no, as it implies we should have feature parity with CRoaring which I guess we cannot commit to. Also if there's a bug in Rust code, we'll have to break the version pin to release a bugfix.

[Dr-Emann]

Yeah, absolutely it doesn't make sense to match on the croaring crate itself.

[Dr-Emann]

Also noticed some functions which should have must_use were missing it, so I went through and checked every function.

[saulius]

Great work @Dr-Emann, thanks for shipping it!