Binary size + composability

[gatesn]

As flagged by @XinyuZeng

Right now it's not possible to strip out unused default encodings from Vortex. For example, if we only include PrimitiveArray in the Context, then many compute function implementations get pulled in from VarbinArray, even with LTO. It's also not clear that we would strip out unused compute functions of used encodings.

This issue is to track a solution to this, likely involving re-architecting our crates / traits.