chore: make conversions between X and Id X explicit in definitions

[eric-wieser]

This is free at runtime, since both Id.run and pure are inlined.

Split from #7352, since this should have minimal impact on downstream code.

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This PR <short changelog summary for feat/fix, see above>.

Closes <RFC or bug issue number fixed by this PR, if any>

[kim-em]

Subsumed by #7352