Add default to TypeFoldable::fold_with

[camsteffen]

TypeFoldable::fold_with delegates to TypeFoldable::try_fold_with with an infallible error type.

[rustbot]

r? @dingxiangfei2009

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.