Defaults, process-on-build and bug fixes

• Support _initializeBuilder hook in value types. If found, it's executed
  when a Builder is created and can be used to set defaults.
• Support _finalizeBuilder hook in value types. If found, it's executed
  when build is called and can be used to apply processing to fields.
• Add defaultCompare and defaultSerialize to @BuiltValue. They
  control the defaults for compare and serialize in @BuiltValueField.
• Add facility to merge Serializers instances via Serializers.merge,
  SerializersBuilder.merge and SerializersBuilder.mergeAll.
• Bug fix: fix generated code with polymorphism and more than one level of
  non-instantiable classes.
• Bug fix: fix generated operator== when a field is called other.
• Fix num deserialization so it does not always convert to double.
• Bump version of analyzer.
• Internal: replace analyzer's DartType.displayName with custom code
  generator.