schemafy_lib: semi-handle patternProperties

[woodruffw]

First of all, thanks for this library!

This is a first-stab attempt at supporting patternProperties, which were silently ignored before (more accurately, resulted in ZSTs).

The new behavior here is to degrade to a BTreeMap if the defined structure contains any patternProperties, since the presence thereof indicates dynamic key names that we can't represent/generate statically. Similarly, we use Value for the value type, since we need to cover everything that might be a valid value under either patternProperties or ordinary properties.

This could be improved slightly by making the value type into an enum of the closure of all possible value types in the actual schema, but I figured I'd start with this.