Make Str.perl 43x faster for some chars

Those that give +uniprop($ord,'Canonical_Combining_Class') > 0
in particular.

The change from lower-case hex to upper-case hex is intentional.
I think it doesn't matter much. We could also skip the hexafication
of ords and simply use the `\c[]` escape, but perhaps decimal
character codes are less familiar to humans reading the output?

src/core/Rakudo/Internals.pm

@@ -722,10 +722,10 @@ my class Rakudo::Internals {
     method PERLIFY-STR(Str \string) {
         sub char-to-escapes(Str $char) {
 #?if moar
-            '\x[' ~ $char.NFC.list.map({ .fmt('%0x') }).join(',') ~ ']'
+            '\x[' ~ $char.NFC.list.map({.base: 16}).join(',') ~ ']'
 #?endif
 #?if !moar
-            $char.ord.fmt('\x[%x]')
+            '\x[' ~ $char.ord.base(16) ~ ']'
 #?endif
         }