flush an objects cache when mixin in

... if this object already is an mixin. Otherwise when mixin in
several things in a row, it would collect precomputed NFAs and
this would result in bigger files when serializing it.
Example: declare 10 infixes, was 9MB .pir file, is now 1.2MB.
         compiling it to pbc took 49s now takes 0.8s.
initialize $!is_mixin in method BUILD