make unbase moderately magic again

This now allows 0x and 0o in :16(), but 0d is correctly interpreted as
a hex byte, not a prefix for decimal. pmichaud++

src/core/Str.pm

@@ -776,7 +776,15 @@ sub trim-trailing(Str:D $s) { $s.trim-trailing }
 
 # the opposite of Real.base, used for :16($hex_str)
 sub unbase(Int:D $base, Str:D $str) {
-    ":{$base}<$str>".Numeric;
+    my Str $prefix = $str.substr(0, 2);
+    if    $base <= 10 && $prefix eq any(<0x 0d 0o 0b>)
+       or $base <= 24 && $prefix eq any <0o 0x>
+       or $base <= 33 && $prefix eq '0x' {
+        $str.Numeric;
+
+    } else {
+        ":{$base}<$str>".Numeric;
+    }
 }
 
 sub chrs(*@c) {