Detect required arg after optional.
Detect required arg after optional.

For now, Clownfish allows required arguments to appear after optional
arguments in a parameter list.  Python does not, so we need to detect
that error case.

Eventually, we'll either fix Clownfish to take on Python's restriction,
or else force the user to create manual glue for problematic methods.

nwellnhof/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,rectang/lucy-clownfish,nwellnhof/lucy-clownfish,apache/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,nwellnhof/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish,rectang/lucy-clownfish,nwellnhof/lucy-clownfish,nwellnhof/lucy-clownfish,rectang/lucy-clownfish,apache/lucy-clownfish