SI-8161 Allow defaults of overloads of different arities

 - The default getter name now includes the (erased) arity of the
   method.
 - Temporarily support the old default getter names at call sites
   to allow us to compile against libraries compiled with the
   old compiler. This is a temporary measure, and doesn't address
   all binary incompatibilies, just enough to get partest-extras
   compiling.