Add fuzzing + CI

[uncomputable]

Fixes #72

Implement fuzzing (Arbitrary) for the Simfony parse tree and write three fuzz targets:

1. compile_text: try to compile random ASCII strings; slow because of parser (#79)
2. compile_parse_tree: try to compile random parse trees; ~10x faster than compile_text and better coverage
3. display_parse_tree: check that Display → ParseFromStr is the identity function

Write a simple CI that uses nix flakes and dev shells. This CI is not more reproducible than regular GitHub CI, but it saves me time because I already have nix infrastructure in place. I will switch the CI to something reproducible in the near future.

Run the linter, unit tests (stable + MSRV) and the fuzzer in CI. For now, coverage is ignored in CI.

[apoelstra]

Neat! I like this "budget" scheme for recursive types.

[uncomputable]

First I had an increasing depth that is checked against a fixed maximum, but then I figured that a budget would be more flexible.