Fix incorrect results in string-returning methods of Rationals

Fixes RT#126016: https://rt.perl.org/Ticket/Display.html?id=126016
The issues observed in the ticket result from floating point math noise when
handling the fractional part after splitting the Rational into whole part and
fractional parts. That fractional part remains a Rat, and when further
operations on it coerce it into a Num, the f.p. math noise results in it being
a whole number, but still treated under the assumption of being less than 1.

Fix by checking for that case and adjusting fractional/whole parts accordingly.
Also, fix Rat.perl.EVAL roundtrip for Rats that end up not being equal to the
original Rat due to f.p. math noise by outputting those in <nu/de> notation.

Performance impact of this commit: Rational.Str about 40% faster,
Rational.base about 30% faster, Rat.perl 5% slower

src/core/Rat.pm

@@ -14,9 +14,12 @@ my class Rat is Cool does Rational[Int, Int] {
                 $d = $d div 2 while $d %% 2;
                 self.REDUCE-ME;
             }
-            $d == 1
-              ?? self.base(10,*)
-              !! '<' ~ $!numerator ~ '/' ~ $!denominator ~ '>'
+            if $d == 1 and (my $b := self.base(10,*)).Numeric === self {
+                $b;
+            }
+            else {
+                '<' ~ $!numerator ~ '/' ~ $!denominator ~ '>'
+            }
         }
     }
 }

src/core/Rational.pm

@@ -72,7 +72,13 @@ my role Rational[::NuT, ::DeT] does Real {
         if nqp::istype($!numerator,Int) {
             my $whole  = self.abs.floor;
             my $fract  = self.abs - $whole;
-            my $result = ($!numerator < 0 ?? '-' !! '') ~ $whole;
+
+            # fight floating point noise issues RT#126016
+            if $fract.Num == 1e0 { $whole++; $fract = 0 }
+
+            my $result = nqp::if(
+                nqp::islt_I($!numerator, 0), '-', ''
+            ) ~ $whole;
 
             if $fract {
                 my $precision = $!denominator < 100_000
@@ -121,11 +127,14 @@ my role Rational[::NuT, ::DeT] does Real {
             $prec = ($!denominator < $base**6 ?? 6 !! $!denominator.log($base).ceiling + 1);
         }
 
-        my $sign   = $!numerator < 0 ?? '-' !! '';
-        my $whole  = self.abs.floor;
-        my $fract  = self.abs - $whole;
-        my $result = $sign ~ $whole.base($base);
+        my $sign  = nqp::if( nqp::islt_I($!numerator, 0), '-', '' );
+        my $whole = self.abs.floor;
+        my $fract = self.abs - $whole;
 
+        # fight floating point noise issues RT#126016
+        if $fract.Num == 1e0 { $whole++; $fract = 0 }
+
+        my $result = $sign ~ $whole.base($base);
         my @conversion := <0 1 2 3 4 5 6 7 8 9
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