Using specialized schema subclasses

[blast-hardcheese]

Resolves #1164

Motivation:
PrimitiveType contains a full mapping (and constructors!) for built-in tracked schema subtypes. By using this, the Schemas generated by the 2to3 converter will have a similar shape to the native 3.x schemas, permitting more unified structure handling for clients (permitting relying on isInstanceOf exclusively, instead of isInstanceOf as well as getType() == "xyz")

Delegating to fromTypeAndFormat to get a particular instance, then createProperty() to instantiate the particular schema resolves the issue I was running into.

If we can't actually look up the mapping, fall back to the original behaviour.

[blast-hardcheese]

I force pushed because I found a fallback case where the base type is recognized by PrimitiveType, but the format was not. I handled this by calling with null as the format, then setFormat on the resulting schema (if it exists) otherwise continuing down the chain to new Schema() with the previous behaviour.

[gracekarina]

thanks @blast-hardcheese