New KeyValuePairs view

[karwa]

TODO:

• Format tests.
• Make sure there are tests for semicolons in content being automatically encoded when using ExtendedForm schema.
• Allow the schema to decide whether non-URL-code-points are escaped. On the one hand, it's not really the schema's business, but on the other hand, it kinda is, because it means those code-points can't be used as an escapable subcomponent delimiter.
• Pin the Apple build-only CI to a specific Xcode version