Remove virtual object fields from IR and compiler

[nomeata]

to build on #529 and remove stuff from the IR.

[nomeata]

Ok, this somewhat complete now. Did not remove the old code yet. The reason it fails is that the front-end still supports vals etc. in object patterns.

[nomeata]

This works now. But there is still code to remove, of course.

@rossberg , do you want to keep val as_obj : value -> value Env.t in the source interpreter, or do a dispatch when interpreting DotE?

[rossberg]

@rossberg , do you want to keep val as_obj : value -> value Env.t in the source interpreter, or do a dispatch when interpreting DotE?

Oh right, the interpreter should do dispatch, too. I overlooked that. Care to add that to this change?

[nomeata]

@rossberg , do you want to keep val as_obj : value -> value Env.t in the source interpreter, or do a dispatch when interpreting DotE?

Oh right, the interpreter should do dispatch, too. I overlooked that. Care to add that to this change?

I’d rather have a separate PR for that.

[rossberg]

Fine as well.

[nomeata]

See #543