Add keyed and derive_key variants for guts

[devinrsmith]

This adds a struct Guts with methods new, new_keyed, and new_derived_key mirroring Hasher construction options.

[oconnor663]

I've been planning a more extensive change here, where the guts module will provide functions that let you initialize or update a regular Hasher in advanced ways. That should support fancy use cases while still taking advantage of the full SIMD/threading performance of Hasher, without needing to reinvent the wheel.

[devinrsmith]

Do you imagine that the guts advanced Hasher would be enough to support the bao implementation, or would bao still need something lower-level?

Any estimate on timeline for the more extensive changes?

I don't expect API stability for guts today; and if you are amenable, would be happy for this to be merged even if it will be replaced in the future.

[oconnor663]

Do you imagine that the guts advanced Hasher would be enough to support the bao implementation

Yes, the goal here would be to at least support:

• Bao as it currently works today
• plus the keyed and derive_key modes, which Bao doesn't currently use but could in theory
• plus a "pre-process the derive_key context string" option

Any estimate on timeline for the more extensive changes?

No I don't have a timeline, but I'll make a calendar note to prioritize it this weekend :)