Auto merge of #50855 - nnethercote:fewer-macro_parser-allocs, r=petrochenkov

Speed up the macro parser

These three commits reduce the number of allocations done by the macro parser, in some cases dramatically. For example, for a clean check builds of html5ever, the number of allocations is reduced by 40%.

Here are the rustc-benchmarks that are sped up by at least 1%.
```
html5ever-check
        avg: -6.6%      min: -10.3%     max: -4.1%
html5ever
        avg: -5.2%      min: -9.5%      max: -2.8%
html5ever-opt
        avg: -4.3%      min: -9.3%      max: -1.6%
crates.io-check
        avg: -1.8%      min: -2.9%      max: -0.6%
crates.io-opt
        avg: -1.0%      min: -2.2%      max: -0.1%
crates.io
        avg: -1.1%      min: -2.2%      max: -0.2%
```

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