Make `Sharded` an enum and specialize it for the single thread case
This changes `Sharded` to use a single shard by an enum, reducing the size of `Sharded` for greater cache efficiency.
Performance improvement with 1 thread and `cfg(parallel_compiler)`:
<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">1.7009s</td><td align="right">1.6748s</td><td align="right">💚 -1.53%</td></tr><tr><td>🟣 <b>hyper</b>:check</td><td align="right">0.2525s</td><td align="right">0.2451s</td><td align="right">💚 -2.90%</td></tr><tr><td>🟣 <b>regex</b>:check</td><td align="right">0.9519s</td><td align="right">0.9353s</td><td align="right">💚 -1.74%</td></tr><tr><td>🟣 <b>syn</b>:check</td><td align="right">1.5504s</td><td align="right">1.5280s</td><td align="right">💚 -1.45%</td></tr><tr><td>🟣 <b>syntex_syntax</b>:check</td><td align="right">5.9536s</td><td align="right">5.8873s</td><td align="right">💚 -1.11%</td></tr><tr><td>Total</td><td align="right">10.4092s</td><td align="right">10.2706s</td><td align="right">💚 -1.33%</td></tr><tr><td>Summary</td><td align="right">1.0000s</td><td align="right">0.9825s</td><td align="right">💚 -1.75%</td></tr></table>
I did see an unexpected 0.23% change for the serial compiler, so this could use a perf run to see if that reproduces.
cc `@SparrowLii`
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