add instance Traversable UrlMap'

[chris-martin]

Before submitting your PR, check that you've:

• Bumped the version number
• Documented new APIs with Haddock markup
• Added @since declarations to the Haddock

After submitting your PR:

• Update the Changelog.md file with a link to your PR
• Check that CI passes (or if it fails, for reasons unrelated to your change, like CI timeouts)

[Vlix]

This looks fine, not a big change or anything.

If you could merge in master so that all the CI works again, we can probably merge this in 🤔

[kazu-yamamoto]

@chris-martin Could you rebase and push -f this branch again?

[chris-martin]

Rebased, checks are looking better.

[Vlix]

CI fails on nightly because of bad constraints on http3. But they would succeed with the correct version of http-semantics.

Since version wai-extra-3.1.15 hasn't been released, please add this change to 3.1.15 in the Changelog (and don't forget to change all the @since declarations).

And to make all the tests pass, change the extra-deps section of stack-nightly.yaml to:

extra-deps:
  - http3-0.0.18
  - network-udp-0.0.0
  - quic-0.2.2
  - sockaddr-0.0.1