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).

Author: TheBlueMatt

ci/check-lint.sh

@@ -2,6 +2,8 @@
 set -e
 set -x
 RUSTFLAGS='-D warnings' cargo clippy -- \
+	`# Things where clippy is just wrong` \
+	-A clippy::unwrap-or-default \
 	`# Errors` \
 	-A clippy::erasing_op \
 	-A clippy::never_loop \