zero-copy support for Batch

[carlsverre]

Is your feature request related to a problem? Please describe.
In my project I am commiting data into Fjall using Batch from a iterator of Bytes. The current Batch's insert method only supports taking values by ref (AsRef<&[u8]>) and hence makes a needless copy into UserValue (type alias to Slice).

Describe the solution you'd like
I'd like to minimize needless copies in my write path. Some possible solutions:

1. modify the insert method to support values that are Into<Slice>. To retain compatibility with existing code this would also require impl<T: AsRef<&[u8]>> From<T> for Slice... which I think should be fine to implement?
2. add a second insert method to batch that supports this kind of zero copy - could be feature flagged under bytes
3. extract commit out of Batch and have it take in an iterator of (&Partition, key, value) such that key/value are Into<Slice>

Describe alternatives you've considered
The current work around is just using batch. But it causes a lot of needless allocation.

Additional context
Happy to brainstorm on this!

[marvin-j97]

Hmm yeah, that makes sense. I think all insert() and remove() methods should probably be Into<Slice> as well...

[carlsverre]

Implementing From<AsRef<&[u8]>> is not trivial to do on the existing Slice implementations due to conflicts with existing (optimized) conversions like From<Vec>. So we could add a NewType that wraps Slice and only implements From<AsRef> and From<Into<Slice>> which... I think will work? And the compiler should eliminate it.

Let me know which general approach you'd prefer and I can implement this.

[marvin-j97]

I think it should just be Into<Slice>, but that is probably a breaking change, but it's kind of hard to say to what extent. Ideally there are enough From<T> impls to catch pretty much all standard use cases, and if not you have to fallback to something like MyType.to_bytes(), which will trigger Slice::From<&[u8]>...

[carlsverre]

Cool, that's pretty easy to do if you're ok with the breaking change. I'll do it this afternoon.