Advanced features, formatting and fixes

• Add @BuiltValue(generateBuilderOnSetField: true) which provides a way to
  listen for set calls on generated builders.
• Add @BuiltValueEnumConst(fallback: true) as a way to mark an enum const
  as the fallback when valueOf or deserialization fails.
• Add @BuiltValueSerializer(serializeNulls: true as a way to modify the wire
  format to explicitly contain null values.
• Make it possible to merge Serializers instances: add a builderFactories
  getter that returns installed builder factories.
• Use new Function syntax everywhere.
• Bug fix: only generate builder factories for fields that are Built
  types or built_collection collections.