This PR upgrades our version of `ctor` to v0.2.6.

We were previously using a fork that contained this fix:
https://github.com/mmastrac/rust-ctor/pull/295.

A new version of `ctor` has now been released with that change, so we
can switch back to the mainline version.

I scanned through the diff between versions (since we're upgrading from
effectively v0.1.20) and didn't notice anything that seemed obviously
breaking:
https://github.com/mmastrac/rust-ctor/compare/564b87f1dc13cf061ef9997ee501fa5378f35ba1...ee6b4b1c7bd3c9c1edad3b6f4d906e767767ba3d

Release Notes:

- N/A