Blake2 missing functionality while migrating from `blake2-rfc`

Not sure if this is a feature request or a question...

I was previously using the blake2-rfc crate and now I want to migrate to the blake2 crate. However previously I was using the Blake2b::with_key constructor (as per https://docs.rs/blake2-rfc/0.2.18/blake2_rfc/blake2b/struct.Blake2b.html#method.with_key) and I can't seem to find the proper equivalent in case of the blake2 crate.

There seems to exist Blake2bVarCore::new_with_params (as per https://docs.rs/blake2/0.10.2/blake2/struct.Blake2bVarCore.html#method.new_with_params), however I can't seem to convert that into a Blake2bVar (which I need to get variable hash output).

Based on the current blake2b implementation, it seems I should be able to just replace the core member of the Blake2bVar with the Blake2bVarCore instance, which indeed works if I "hack my way" by using pointers. However there is no "official" (as per the public API) way of doing this.

A simple From implementation, or even an unsafe constructor that takes a Blake2bVarCore and yields a Blake2bVar would seem to suffice.

Start with the current blake2b implementation, then inspect Blake2bVarCore::new_with_params and the Blake2bVar API. Determine how a configured core can be exposed through the public variable-output type without relying on pointer hacks. Done means callers migrating from blake2-rfc can construct a keyed variable-output Blake2b hasher through a safe, documented public API.