Async support

[petrnymsa]

It would be nice to support async mapping, such as "custom" on Field can return Future.

This proposal needs to be specified

[tenhobi]

This should be quite easy I hope. Just check whether the function is async and if true, add an await in front of it.

BUT we would need an async version for all of convert, tryConvert, ... Damn, probably even all the functions that use the one mapping...

The issue here is that async is contagious.

[tenhobi]

Can we somehow process futures synchronously?

I found https://api.dart.dev/stable/2.3.1/dart-cli/waitFor.html which should work in CLI.