add BackTrace class to setting. Not yet used in exceptions

src/core/BackTrace.pm

@@ -0,0 +1,42 @@
+my $*VERBOSE_BACKTRACE = False;
+class BackTraceLine {
+    has Str $.file;
+    has Int $.line;
+    has Str $.subname;
+    has     $.subtype;
+
+    multi method Str(BackTraceLine:D:) {
+        my $s = $.subname // '<anon>';
+        my $what = lc $.subtype.perl;
+        "  in $what $s at {$.file}:$.line"
+    }
+}
+
+class BackTrace is List {
+    method new(Int $offset = 1) {
+        my $new = self.bless(*);
+        my $bt = nqp::atkey(pir::getinterp, 'context').backtrace;
+        for $offset .. $bt.elems - 1 {
+            next if pir::isnull($bt[$_]<sub>);
+            my Mu $p6sub =
+                pir::perl6_code_object_from_parrot_sub__PP($bt[$_]<sub>);
+            next if !$*VERBOSE_BACKTRACE && !$p6sub.defined;
+            next if !$*VERBOSE_BACKTRACE && !$p6sub ~~ Routine;
+            my $line     = $bt[$_]<annotations><line>;
+            my $file     = $bt[$_]<annotations><file>;
+            my $subname  = nqp::p6box_s($bt[$_]<sub>);
+            $subname = '<anon>' if $subname.substr(0, 6) eq '_block';
+            $new.push: BackTraceLine.new(
+                :$line,
+                :$file,
+                :$subname,
+                :subtype($p6sub.WHAT),
+            );
+        }
+        $new;
+    }
+
+    multi method Str(BackTrace:D:) {
+        self.join("\n") ~ "\n"
+    }
+}

tools/build/Makefile.in

@@ -164,6 +164,7 @@ CORE_SOURCES = \
   src/core/Complex.pm \
   src/core/Exception.pm \
   src/core/Failure.pm \
+  src/core/BackTrace.pm \
   src/core/Exceptions.pm \
   src/core/EXPORTHOW.pm \
   src/core/operators.pm \