fix: properly gate things by features & test for that

[danking]

Ideally we would test every combination of features, but at the very least, we should verify that no features, default features, and all features pass clippy.

The two main changes:

1. In vortex-io, we need a syntactically valid expressoin when neither compio nor tokio are available.
2. In vortex-array, we need both flatbuffers and flexbuffers for views to work. I removed the optional dependencies on these libraries. In theory, someone could go through our code base and properly feature flag array views, but that began to feel onerous as I did it.

[joseph-isaacs]

We can use compile time errors https://github.com/spiraldb/vortex/pull/1496