In the route syntax, allow trailing backslashes to indicate line continuation.

[whittle]

Just as a convenience to those of us who have type identifiers in our routes that are very long and bound to the database schema.

Before submitting your PR, check that you've:

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

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)

[snoyberg]

LGTM

[snoyberg]

Thanks!