Allow a wider range of input keying

[tgolsson]

The current input with zero-copy of the keys is very nice from a performance view, but it requires users to jump through hoops if it's not a 100% direct call (e.g. some of our internal incremental FFI patterns).

Having multiple ways of feeding data (borrowed key, owned key, indexed key?) would be neat in order to make this a bit more ergonomic.

Rabbit holes: How does this work if we also want to support other input types like i32 in the future? (Enum explosion?)