core: find replacement for .sketches()?

[bluegenes]

per @luizirber (paraphrased) -- we should find a replacement for .sketches as it clones all sketch data

https://github.com/sourmash-bio/sourmash/blob/latest/src/core/src/signature.rs#L480-L482

 pub fn sketches(&self) -> Vec<Sketch> {
        self.signatures.clone()
    }

Note: sig.iter() should work for most functions where we might want to use .sketches()?