Implement PartialEq and Eq for Ident, Punct and Span

[c410-f3r]

Related #154 and #1320.

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[petrochenkov]

No PartialEq/Eq for Ident and Span please, they cause bugs.
See, for example, #58942 as the most recent instance.
Some explicit way to compare only span locations or only Ident's string representations would be ok, I guess.

Punct also contains a span, so comparisons are at least questionable, and most likely wrong if they are derived.

[petrochenkov]

cc #51074

[c410-f3r]

Thanks for the explanation @petrochenkov