Avoid constructing rust strings when performing varbin/varbinview dict compression

[robert3005]

Dict compression does a very naive iteration of the array which constructs rust strings from the array data. Since we only need to hash the bytes we can avoid constructing those strings and we can avoid wrapping them in options.

https://github.com/spiraldb/vortex/blob/develop/encodings/dict/src/compress.rs#L88-L107

related to #1453

[robert3005]

We do not construct rust strings, we always operate on byte slices