chore: Clippy lints, tests

See merge request !4

Update to current stable rust edition 2021 and remove unneeded features

See merge request redox-os/posix-regex!3

Spring cleanup

See merge request !1

Apparently the end marker thingy isn't needed, because all nodes, even
optional ones, will be explored to the final node anyway I realized.

This prepares for a future data structure that will share data between
nodes, like an append-only vector. It's a little slower for now, but I
bet it'll be faster soon :D

This is a pretty big speedup, and I intend to keep optimizing it until
the code sucks less.