Track build time.
Moves comment.
Disable langbs on aarch64-linux, until it works there.
Adds start of levitation docs, the start of converting expressions to Core.
Enables aarch64-linux, since the default output works.
Don't make the output a linkFarm, instead copy everything, making it symlink-free.
Add flake support to build.
Update builds.sr.ht file.
Adds Hydra job.
Doc reorg.

This is kind of a checkpoint, because I'm gonna switch to flakes in a
second; evaluation is taking _way_ too long for some reason, and caching
would be nice.
Update for QuickJS version bump in nixpkgs.
Removes mdbook-katex, 'cause it was failing the build and only used in one place.
Allow a couple warnings.
Adds push-enter machine.
Adds $ form to explicitly specify application.
Printing improvements, tuple form.
More proof work.
Experiment in ZAM-style push-enter machines.