Properly support propertyNames #396
Description
The current implementation of propertyNames is completely separate from the remaining validation code, implementing special cases for stuff like minLength.
According to the spec, however, propertyNames should be treated like a normal json schema being applied to a text node of the property value.
This has lead to issues, such as #342 and #326.
I propose to just do what the spec says and apply the node content under propertyNames to each field name as if it were a TextNode.