spec shaking v2: Preserve spec entries with `export = true`

[mootz12]

What version are you using?

main (commit 7f894f4)

What did you do?

Define a #[contracttype(export = true)] and #[contracterror(export = true)] that were not used at the boundary of the contract.

What did you expect to see?

These types don't get shaken and exist in the spec.

What did you see instead?

The types got shaken and did not exist in the spec. That behavior is surprising. If a user explicitly opts a type into export, it should be retained after spec shaking.

[tupui]

Thanks for following up on that!