Fix Issue #109 (no error for Multiple inheritance)

Fix issue #109 where incorrect errors are being displayed when you try and inherit from multiple classes.

Instead of the compiler errors "expected {" and "expected ;" the compiler now displays the proper error, "A class can only extend a single class". A side effect of what I did also caused the following changes which made me update a bunch of the baselines:

class a extends b, { } 
used to have the error "{ expected" but now says "Trailing comma not allowed"

class a extends void {}
used to have the error "Identifier expected" but now says "Type Reference expected"

class a extends {}
user to have the error "Identifier expected" but now the compiler doesn't see this as an error (in this case I wasn't sure if I should add a new error message for an empty extends clause or not)

To me these error messages seem to make more sense but it worries me that I had to alter so many baselines. If these new baselines are incorrect let me know and I can figure out a way to rewrite my fix.