Rollup merge of #154419 - zetanumbers:take-first-group, r=nnethercote

Take first task group for further execution

Continuing from https://github.com/rust-lang/rust/pull/153768#discussion_r2938828363.

I thought that storing a first group of tasks for immediate execution instead of pushing and immediately poping it from rayon's local task queue in par_slice would avoid overwhelming work stealing potentially blocking the original thread. So I've implemented this change.

8 threads benchmarks:

<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>baseline~~9</b></th><td colspan="2"><b>new~take-first-group~1</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.1110s</td><td align="right">0.1086s</td><td align="right">💚  -2.13%</td></tr><tr><td>🟣 <b>hyper</b>:check:initial</td><td align="right">0.1314s</td><td align="right">0.1298s</td><td align="right">💚  -1.23%</td></tr><tr><td>🟣 <b>hyper</b>:check:unchanged</td><td align="right">0.0771s</td><td align="right">0.0755s</td><td align="right">💚  -2.14%</td></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">0.3787s</td><td align="right">0.3757s</td><td align="right"> -0.80%</td></tr><tr><td>🟣 <b>clap</b>:check:initial</td><td align="right">0.4680s</td><td align="right">0.4564s</td><td align="right">💚  -2.48%</td></tr><tr><td>🟣 <b>clap</b>:check:unchanged</td><td align="right">0.2337s</td><td align="right">0.2301s</td><td align="right">💚  -1.52%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">0.4321s</td><td align="right">0.4265s</td><td align="right">💚  -1.31%</td></tr><tr><td>🟣 <b>syn</b>:check:initial</td><td align="right">0.5586s</td><td align="right">0.5401s</td><td align="right">💚  -3.31%</td></tr><tr><td>🟣 <b>syn</b>:check:unchanged</td><td align="right">0.3434s</td><td align="right">0.3429s</td><td align="right"> -0.14%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.2755s</td><td align="right">0.2661s</td><td align="right">💚  -3.40%</td></tr><tr><td>🟣 <b>regex</b>:check:initial</td><td align="right">0.3350s</td><td align="right">0.3347s</td><td align="right"> -0.11%</td></tr><tr><td>🟣 <b>regex</b>:check:unchanged</td><td align="right">0.1851s</td><td align="right">0.1832s</td><td align="right">💚  -1.01%</td></tr><tr><td>Total</td><td align="right">3.5296s</td><td align="right">3.4695s</td><td align="right">💚  -1.70%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9837s</td><td align="right">💚  -1.63%</td></tr></table>