fix compilation error with async-std

Compilation with async-std is currently broken

$> cd crates/arti
$> cargo check --no-default-features --features async-std,native-tls
    Checking arti v1.0.0 (/home/trinity/dev/tor/core/arti/crates/arti)
error[E0433]: failed to resolve: could not find `consts` in `s`
  --> crates/arti/src/process.rs:92:51
   |
92 |             let mut signal = s::Signals::new(&[s::consts::signals::SIGHUP])?;
   |                                                   ^^^^^^ could not find `consts` in `s`

error[E0425]: cannot find value `signals` in this scope
  --> crates/arti/src/process.rs:93:16
   |
93 |             Ok(signals.map(|i| i*2))
   |                ^^^^^^^ help: a local variable with a similar name exists: `signal`

warning: unused import: `error`
   --> crates/arti/src/lib.rs:268:15
    |
268 | use tracing::{error, info, warn};
    |               ^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

warning: unused import: `futures::stream::StreamExt`
  --> crates/arti/src/process.rs:91:17
   |
91 |             use futures::stream::StreamExt as _;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^

Some errors have detailed explanations: E0425, E0433.
For more information about an error, try `rustc --explain E0425`.
warning: `arti` (lib) generated 2 warnings
error: could not compile `arti` due to 2 previous errors; 2 warnings emitted

I should get going on #303 (closed); this shouldn't have ship as part of 1.0.0, but it did 😕