Fix user and kernel times on Windows

[clemenswasser]

Description

By using a job object and attaching it to the spawned process before launch, we can record the total user and kernel times of it and all of its child processes. Sadly extracting the raw handle of the main_thread_handle is still nightly only 😢 (see: rust-lang/rust#96723) and required, in order to resume a initially suspended process. Therefore this new CPUTimer implementation is guarded behind a new nightly cargo feature and not enabled by default.

Example

Before:

> cargo run --quiet -- 'cargo fmt'
Benchmark 1: cargo fmt
  Time (mean ± σ):     175.6 ms ±   1.1 ms    [User: 0.9 ms, System: 8.4 ms]
  Range (min … max):   173.3 ms … 177.3 ms    16 runs

After:

> cargo run --quiet -- 'cargo fmt'
Benchmark 1: cargo fmt
  Time (mean ± σ):     175.8 ms ±   1.9 ms    [User: 91.5 ms, System: 62.5 ms]
  Range (min … max):   173.5 ms … 180.7 ms    16 runs

Fixes: #368