detect ambiguities to same rule

from Rats! grammar:

declarationOrStatement
  : declaration
  | statement
  ;

where declaration reaches

blockDeclaration :
  ( 'static' )? block
  ;

Can do a breadth first search after-the-fact or might be able to do
something with the rules we detected during recursion checking in
closure(). Add a simple check for left-factorability too. Showing
the ambiguity would be easy enough here to block and would be awesome!