[operations] Support for optional / default value

[paraboul]

Example here : https://chromium.googlesource.com/chromium/blink/+/master/Source/core/xmlhttprequest/XMLHttpRequest.idl#62

boolean foobar(long x, optional int last = 42);

This should create only one C++ virtual function with 2 arguments.
The overload resolution algorithm must check (in that case) for 1 and 2 arguments.

This means that a similar prototype with one argument is not valid. e.g. :

boolean foobar(long x, optional int last = 42);
boolean foobar(long x); // WRONG