bitpacking `is_constant` is inefficient

[joseph-isaacs]

We could improve the is_constant check for bitpacking.

By checking is all patches and equal and all chunks are equal.

[robert3005]

I remember I didn't do it originally because you have to make sure to override all the patched/invalid values in the underlying buffers with a constant value. It could be because we copy that we initialise them to 0 but should check as part of this work