Summary:
Adds:
- callAsConstructor
- Ability to provide `this` object to callAsFunction
- getPropertyNames() now returns Strings (which can be converted by the caller to std::string if they want).
Fixes:
- double free issue with the String move constructor
Reviewed By: lexs
Differential Revision: D3515398
fbshipit-source-id: afa1342044e41fdd833dd27b8a244a58d4078442