Debug.Assert calls and debug messages

[y-lohse]

After the typescript switch, we'll have a Debug.ts file (see this comment).

We should make sure all the calls from the C# versions are here.

At the same time, we should figure out how to handle logs. There are a few places where we need to print out warnings (eg. version mismatches), but since inkjs can reasonably used as a command line tool it maybe shouldn't write stuff like that along with the regular output. Maybe we can have a toggle flag somewhere.