feat: cost of for is ~0

We fairly often perform FoR and then do not perform bitpacking because
we have exceeded the depth. FoR is very cheap and only worthwhile in
combination with bitpacking anyway, so setting its cost to zero should
improve compression ratio with a minor decrease in compresssion speed.

vortex-sampling-compressor/src/compressors/for.rs

@@ -20,6 +20,10 @@ impl EncodingCompressor for FoRCompressor {
         FoR::ID.as_ref()
     }
 
+    fn cost(&self) -> u8 {
+        0
+    }
+
     fn can_compress(&self, array: &Array) -> Option<&dyn EncodingCompressor> {
         // Only support primitive arrays
         let parray = PrimitiveArray::try_from(array).ok()?;