Fix exponentiation by large negative Ints...

and speedup exponentiation by any sized negative Ints. The fix is to
revert the incorrect underflow check added in 6f6fd1f and the speedup
comes from bypassing `&infix:</>` when creating the `1 / $power` Rational
since we have some knowledge about the values. Also switch to use
`.Failure` on the exceptions for smaller bytecode (though `&infix:<**>`
is still too large to inline).

src/core.c/Int.pm6

@@ -381,23 +381,21 @@ multi sub infix:<%%>(int $a, int $b --> Bool:D) {
     nqp::hllbool(nqp::iseq_i(nqp::mod_i($a, $b), 0))
 }
 
-my constant UINT64_UPPER = nqp::pow_I(2, 64, Num, Int);
-
 multi sub infix:<**>(Int:D $a, Int:D $b --> Real:D) {
     my $power;
     if nqp::isge_I($b, 0) {
         $power := nqp::pow_I($a, $b, Num, Int);
         # when a**b is too big nqp::pow_I returns Inf
         nqp::istype($power, Int)
             ?? $power
-            !! Failure.new(X::Numeric::Overflow.new)
+            !! X::Numeric::Overflow.new.Failure
     }
     else {
         $power := nqp::pow_I($a, nqp::neg_I($b, Int), Num, Int);
         # when a**b is too big nqp::pow_I returns Inf
-        nqp::istype($power, Num) || (nqp::isge_I($power, UINT64_UPPER) && nqp::isne_I($a, 0))
-            ?? Failure.new(X::Numeric::Underflow.new)
-            !! 1 / $power
+        nqp::istype($power, Num) || (($power := CREATE_RATIONAL_FROM_INTS(1, $power, Int, Int)) == 0 && nqp::isne_I($a, 0))
+            ?? X::Numeric::Underflow.new.Failure
+            !! $power
     }
 }
 

src/core.c/Rat.pm6

@@ -31,6 +31,8 @@ my class Rat is Cool does Rational[Int, Int] {
     }
 }
 
+my constant UINT64_UPPER = nqp::pow_I(2, 64, Num, Int);
+
 my class FatRat is Cool does Rational[Int, Int] {
     method FatRat(FatRat:D:) { self }
     method Rat(FatRat:D:) {