Better support for application/x-www-form-urlencoded bodies.

[jwalton]

Exegesis has support for x-www-form-urlencoded bodies, but if you give it a complicated schema, it will probably have a hard time finding your properties, or may get the types wrong.

You can avoid this by using a schema where all the properties are at the top level. But what really needs to happen here is, we need to replace this with something which recursively traverses all the allOf, oneOf, and anyOf keys to extract a schema for each property.