do not use a nested class for Backtrace::Frame (take 2)

mostly so that .WHAT gives the full name

Now without infinite recursion in the backtrace printer
(contains a workaround for RT #114034)

src/core/Backtrace.pm

@@ -1,28 +1,30 @@
 my class Exception { ... }
 
+my class Backtrace { ... }
 
-my class Backtrace is List {
-    class Frame {
-        has Str $.file;
-        has Int $.line;
-        has Mu  $.code;
-        has Str $.subname;
-
-        method subtype(Frame:D:) {
-            my $s = $!code.^name.lc.split('+', 2)[0];
-            $s eq 'mu' ?? '' !! $s;
-        }
+my class Backtrace::Frame {
+    has Str $.file;
+    has Int $.line;
+    has Mu  $.code;
+    has Str $.subname;
 
-        multi method Str(Backtrace::Frame:D:) {
-            my $s = self.subtype;
-            $s ~= ' ' if $s.chars;
-            "  in {$s}$.subname at {$.file}:$.line\n"
-        }
+    method subtype(Backtrace::Frame:D:) {
+        my $s = $!code.^name.lc.split('+', 2)[0];
+        $s eq 'mu' ?? '' !! $s;
+    }
 
-        method is-hidden(Frame:D:)  { $!code.?is_hidden_from_backtrace }
-        method is-routine(Frame:D:) { $!code ~~ Routine }
-        method is-setting(Frame:D:) { $!file eq 'src/gen/CORE.setting' }
+    multi method Str(Backtrace::Frame:D:) {
+        my $s = self.subtype;
+        $s ~= ' ' if $s.chars;
+        "  in {$s}$.subname at {$.file}:$.line\n"
     }
+
+    method is-hidden(Backtrace::Frame:D:)  { $!code.?is_hidden_from_backtrace }
+    method is-routine(Backtrace::Frame:D:) { $!code ~~ Routine }
+    method is-setting(Backtrace::Frame:D:) { $!file eq 'src/gen/CORE.setting' }
+}
+
+my class Backtrace is List {
     proto method new(|$) {*}
 
     multi method new(Exception $e, Int $offset = 0) {