Fix parsing of denormal Nums in &val

We can't just raise 10 into a power to a magnitude of a Num,
as denormals' magnitude would not fit into a normal double.

Fix by using an Int instead and then producing the Num with
nqp::div_In op. This aligns the parsing in &val with the
parsing done in the grammar for literal numbers.

src/core/allomorphs.pm6

@@ -367,11 +367,20 @@ multi sub val(Str:D $MAYBEVAL, :$val-or-fail) {
                     if nqp::iseq_i($p, -1);
                 $pos    = $p;
 
-                return nqp::p6box_n(nqp::mul_n(
-                    $frac ?? nqp::add_n( $int.Num, nqp::div_n($frac.Num, $base.Num) )
-                          !! $int.Num,
-                    nqp::pow_n(10e0, nqp::atpos($parse, 0).Num)
-                )) # if we have a zero, handle the sign correctly
+                my $power := nqp::pow_I(10,
+                  nqp::abs_I(nqp::atpos($parse, 0), Int), Num, Int);
+
+                if $frac {
+                    $int := nqp::add_I(
+                      nqp::mul_I($int, $base, Int), $frac, Int);
+                }
+                else {
+                    $base := 1;
+                }
+                return (nqp::islt_I(nqp::atpos($parse, 0), 0)
+                  ?? nqp::div_In($int, nqp::mul_I($base, $power, Int))
+                  !! nqp::div_In(nqp::mul_I($int, $power, Int), $base)
+                ) # if we have a zero, handle the sign correctly
                 || nqp::if(nqp::iseq_i($neg, 1), -0e0, 0e0);
             }