fix error with named lifetimes in #[new] return types

[Icxolu]

While exploring I noticed that #[new] functions with named lifetimes in their output type give a compile error. This is because the type is used in our const specialization. There is no particular reason to block such types, most notable Bound is one of them, so this fixes that.

[codspeed-hq]

Merging this PR will not alter performance

✅ 105 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing Icxolu:fix-new-bound (fca4b21) with main (4712a0a)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[davidhewitt]

Thanks!