Auto merge of #108693 - Zoxc:arena-opt-funcs, r=cjgillot

Optimize DroplessArena arena allocation

This optimizes `DroplessArena` allocation by always ensuring that it is aligned to `usize` and adding `grow_and_alloc` and `grow_and_alloc_raw`functions which both grow and allocate, reducing code size.

<table><tr><td rowspan="2">Benchmark</td><td colspan="1"><b>Before</b></th><td colspan="2"><b>After</b></th></tr><tr><td align="right">Time</td><td align="right">Time</td><td align="right">%</th></tr><tr><td>🟣 <b>clap</b>:check</td><td align="right">1.6968s</td><td align="right">1.6887s</td><td align="right"> -0.48%</td></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.2552s</td><td align="right">0.2551s</td><td align="right"> -0.03%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.9613s</td><td align="right">0.9553s</td><td align="right"> -0.62%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">1.5402s</td><td align="right">1.5374s</td><td align="right"> -0.18%</td></tr><tr><td>🟣 <b>syntex_syntax</b>:check</td><td align="right">5.9175s</td><td align="right">5.8813s</td><td align="right"> -0.61%</td></tr><tr><td>Total</td><td align="right">10.3710s</td><td align="right">10.3178s</td><td align="right"> -0.51%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9962s</td><td align="right"> -0.38%</td></tr></table>

Author: bors