This makes the code shorter and clearer.
The only incompatible change is that an explicit command-line argument
`--crates-toml=` will take precedence over the `LINTCHECK_TOML`
environment variable.
Using `rayon::current_num_threads()` causes a bug:
```
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
ThreadPoolBuildError { kind: GlobalPoolAlreadyInitialized }',
src/main.rs:632:10
```
Moreover, using the number of threads and dividing it by 2 wouldn't
return the number of physical threads on modern processors which have
a varying number of threads per core.
It makes little sense to restrict ourselves to physical threads,
especially when, in modern architectures, cores with multiple threads
are often faster (performance) while cores with a unique threads are
often slower (efficient). The Rust runtime will make a better choice.
