Use UTF-8 strings

[greenrobot]

Native ObjectBox uses UTF-8, while Dart's String uses UTF-16.

Check our CString wrapper Dart class, which should do proper conversion.

PS.: Is there a better way to process chars than iterating over each character? E.g. is there a byte buffer, we could interpret as UTF-8? If FFI does not offer that yet, please check if they already have an issue for that (or create one).

[greenrobot]

See PR #14

We should still investigate if we can get a better Cstring into FFI