Switch Int to use bigint. Note that we didn't switch to any of the bi…

…gint ops yet, though since bigint knows how to box/unbox native ints, we largely get away with it. New failures are confined to tests that have some clear relation to numerics, and even then the fallout doesn't immediately seem too bad.

src/Perl6/Metamodel/BOOTSTRAP.pm

@@ -558,9 +558,10 @@ Real.HOW.add_parent(Real, Numeric);
 #     has int $!value is box_target;
 #     ...
 # }
+my knowhow bigint is repr('P6bigint') { }
 my stub Int metaclass Perl6::Metamodel::ClassHOW { ... };
 Int.HOW.add_parent(Int, Real);
-Int.HOW.add_attribute(Int, BOOTSTRAPATTR.new(:name<$!value>, :type(int), :box_target(1)));
+Int.HOW.add_attribute(Int, BOOTSTRAPATTR.new(:name<$!value>, :type(bigint), :box_target(1)));
 Int.HOW.set_boolification_mode(Int, 1);
 Int.HOW.publish_boolification_spec(Int);