Increase 64-bit rotation from 20 to 26 bits on finalizer

[lsunsi]

The current finalizer consisting of a right-rotation of 20 bits can create fairly catastrophic results if the bottom input bits are low-entropy and the hash table grows beyond 2^20 elements, see rust-lang/rust#135477 (comment).

In this PR I bumped the number of bits to 26 from 20 in order to allow for bigger hash tables. This is a palliative solution because it increases the chance of collisions from what I can gather. The proper solution that would render the whole rotention irrelevant would be to fix the hashbrown implementation in a way I could not describe here but @orlp and @Noratrieb definitely could if you ask them haha.

Further, I changed the tests in an ad-hoc manner because I couldn't do it otherwise, so if it makes sense to do it in another way please let me know.

Also, I changed the version number to 2.1.1 because the only change was a private constant that should not implicate anything anywhere, but of course if 2.2.0 makes more sense I'll change it right away.

If any other change is required, please let me know!

[Noratrieb]

thanks! I'll merge and release this later today (if I forget please ping me again)

[niebayes]

@Noratrieb While rustc_hash is not marked as stable, introducing breaking changes in a minor version is quite disruptive. The recent rotation change has caused unintended issues with our database project. It would be preferable to reserve breaking changes for major version releases.

[Noratrieb]

The primary intended purpose of this crate is hashmap hashing, where stability doesn't really matter. I don't think this crate should be used for anything else, it's a very bad quality hash. That could probably be better noted in the readme.