Publish some crates to crates.io

[nspin]

It's time to revisit #1.

Aside from some remaining work to nail down, stabilizing, and documenting a few remaining public APIs, here are some questions that must be answered:

Question: How will releases of these crates relate to seL4 releases?

Tentative answer: each release of a (seL4-dependant) crate from this project is only guaranteed to work with one seL4 release. After a seL4 release, development on this project will move to that seL4 release and never look back.

Question: What seL4 configurations will we use to render docs for docs.rs?

Tentative answer: for each relevant target triple supported by docs.rs, we will choose one seL4 configuration. This configuration will have some features like virtualization enabled where relevant.

Question: Should seL4 configurations used for docs.rs use MCS or not?

No tentative answer yet.