Using an enum to represent all of the base simple types defined in XML
Schema.  All the others are just derivations.
Should add in ANYSIMPLETYPE if someone derives from it directly.
Removing the Avro and JSON type information from XmlSchemaTypeInfo.
These are Avro-specific and will be added later in the chain.
Updating the AvroSchemaGenerator to use recognized types, not avro
types, from XmlSchemaTypeInfo, and also to support mixed types, and
correctly support optional types.
Removing last of calls to getAvroType().
Moving over XML Schema JSON representation as well.
Fixed bugs; TestAvroSchemaGenerator passes.