`KDF` trait

[jschneider-bensch]

Should be implemented in

• libcrux-hkdf
• blake2

[github-actions]

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[franziskuskiefer]

Putting this into #1107 as part of the API work.

[franziskuskiefer]

Let's make sure they are consistent in usage and have documentation

[keks]

notes https://md.cryspen.com/p35qd5RXSxqbXc9BwS43Vg#

[keks]

After a bit of back and forth, we decided that it doesn't really make sense to offer a trait here because the interfaces of blake2 and hkdf are so different. Instead, we are improving the respective APIs.