Backcompat mode

[darobin]

The code now needs to be updated to add a few things from WebIDL that are missing, but the question of what to do with old constructs needs to be handled.

My proposal is this: keep them around, but for each of those when they are processed make a call to checkCompatMode(). If compatibility has been set, then accept the old constructs, otherwise die with a syntax error.

This is useful because it will make it possible to parse older specifications, warn, etc.