Improve --jobs UI

[carolynzech]

Improve the documentation for the --jobs option, change it to use hide_short_help like other unstable options, and print the current thread number so that users can match the harness to its output.

Example:

#[kani::modifies(x)]
#[kani::ensures(|_| x.iter().map(|v| *v == 0).fold(true,|a,b|a&b))]
fn zero(x: &mut [u8]) {
    x.fill(0)
}

#[kani::proof_for_contract(zero)]
fn harness() {
    let mut x = [kani::any(), kani::any(), kani::any()];
    zero(&mut x);
}

#[kani::proof]
fn harness_2() {
    assert!(true);
}

#[kani::proof]
fn harness_3() {
    assert!(false);
}

cargo kani -j --enable-unstable --output-format=terse -Z function-contracts
Kani Rust Verifier 0.57.0 (cargo plugin)
warning: Found the following unsupported constructs:
             - caller_location (1)
             - foreign function (1)
         
         Verification will fail if one or more of these constructs is reachable.
         See https://model-checking.github.io/kani/rust-feature-support.html for more details.

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s
Thread 4: Checking harness harness_2...
Thread 2: Checking harness harness_3...
Thread 4: 
VERIFICATION RESULT:
 ** 0 of 1 failed

VERIFICATION:- SUCCESSFUL
Verification Time: 0.0137195s

Thread 2: 
VERIFICATION RESULT:
 ** 1 of 1 failed
Failed Checks: assertion failed: false
 File: "src/lib.rs", line 26, in harness_3

VERIFICATION:- FAILED
Verification Time: 0.012914542s

Thread 0: Checking harness harness...
Thread 0: 
VERIFICATION RESULT:
 ** 0 of 217 failed

VERIFICATION:- SUCCESSFUL
Verification Time: 18.47224s

Summary:
Verification failed for - harness_3
Complete - 2 successfully verified harnesses, 1 failures, 3 total.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[zhassan-aws]

Thanks!