added generated function enum_to_string

[ctin]

I extended c++ generator by adding string representation of enum values.
Rules:

• now generated enum will have cpp file with a function body;
• serialize function named $self_to_string, where $self is generated enum name;
• function name depends on user style;
• function returns std::string or std::wstring depending on djinni option cpp-use-wide-strings

tests passed
test makefile changed to support Xcode 11

[artwyman]

Seems reasonable, but this repo isn't taking any new feature enhancements, per the note in the README:

Maintenance note: This repo is stable but no longer actively maintained. No further development is planned, and no new feature PRs will be merged. Bug fixes may be merged on a volunteer basis. If you have questions or want to talk to other users of Djinni, you can join the Slack community via the link at the end of this document.