Import the WebIDL parser from Gecko.
Remove support for external interfaces.
Reinstate IDLCallbackType.module.
Add support for undefined default values for 'any'.

This also updates TestBinding to take into account the automatic default
for optional 'any' arguments and dictionary members.
Remove unsupported extended attributes from IDL.

These were silently ignored so far.
auto merge of #2611 : Ms2ger/servo/webidl-parser-sync, r=jdm