Use state machine to parse directives

[djc]

Intended to fix #3174. Have not done any benchmarks yet -- suggestions on how best to approach that are welcome.

This is a little bit more code, but IMO maybe slightly more readable?

It's correct enough that it passes the tests, there might be edge cases that aren't covered but those should be easy to solve.

[dpc]

> hyperfine --warmup 30 -N ./target/release/dotr-before ./target/release/dotr-after
Benchmark 1: ./target/release/dotr-before
  Time (mean ± σ):       1.2 ms ±   0.1 ms    [User: 0.4 ms, System: 0.7 ms]
  Range (min … max):     1.1 ms …   1.9 ms    2044 runs

Benchmark 2: ./target/release/dotr-after
  Time (mean ± σ):     613.8 µs ±  79.6 µs    [User: 289.1 µs, System: 254.0 µs]
  Range (min … max):   511.5 µs … 979.8 µs    5264 runs

Summary
  ./target/release/dotr-after ran
    2.03 ± 0.34 times faster than ./target/release/dotr-before 

Added true just to have a baseline:

Benchmark 1: ./target/release/dotr-before
  Time (mean ± σ):       1.2 ms ±   0.1 ms    [User: 0.4 ms, System: 0.7 ms]
  Range (min … max):     1.1 ms …   1.8 ms    2535 runs

Benchmark 2: ./target/release/dotr-after
  Time (mean ± σ):     611.7 µs ±  80.3 µs    [User: 295.6 µs, System: 244.1 µs]
  Range (min … max):   518.8 µs … 1663.5 µs    3825 runs

Benchmark 3: true
  Time (mean ± σ):     595.8 µs ±  74.1 µs    [User: 311.4 µs, System: 215.4 µs]
  Range (min … max):   512.3 µs … 1035.5 µs    5362 runs

This app sets RUST_LOG internally, so setting it from the outside doesn't make a difference.

AFAICT you fixed it and it's nearly free now.

[klensy]

After this, regex can be removed from Cargo.toml (for tracing-subscriber), only left in dev-deps?

[djc]

After this, regex can be removed from Cargo.toml (for tracing-subscriber), only left in dev-deps?

Nice catch, yes! Added that into the first commit.