Allow `clippy::unwrap-or-default` because its usually wrong

Allow `clippy::unwrap-or-default` because its usually wrong

`or_default` is generally less readable than writing out the thing
we're writing, as `Default` is opaque but explicit constructors
generally are not. Thus, we ignore the clippy lint (ideally we
could invert it and ban the use of `Default` in the crate entirely
but alas).