[6.d] Attach ex. handler to start in sink context

Per 6.d-prep: https://github.com/perl6/6.d-prep/blob/69a7d9c5e5d8ff3bed69c6f33577ada4bb8470be/TODO/FEATURES.md#make-start-blocks-in-sink-context-attach-an-error-handler

I've used the normal handler instead of `uncaught_handler` the 6.d-prep
notes say because I don't know how to use the latter (and noticed that
note too late). I'm also unsure if the uncaught_handler handles custom
exception handlers; this commit does.

When detecting the `start` was in sink context, we create a thunk
with the call to original block and use the `handle` op to handle the
exception. The thunk is there so the user can use their own `CATCH`.

One problem that I noticed is code like `sub foo { start die }()`
does NOT attach the handler, because sinkage of stuff like that
requires calls to `.sink` method instead of the `unwanted` Actions helper.

src/Perl6/Actions.nqp

@@ -325,6 +325,21 @@ sub unwanted($ast, $by) {
             $ast.sunk(1);
         }
         elsif $ast.op eq 'callmethod' {
+            if $ast.has_ann('promise_starter') && ! $*W.lang-ver-before('d') {
+                $ast[1] := QAST::WVal.new: value =>
+                  $*W.find_symbol(['&trait_mod:<is>'])(:hidden-from-backtrace,
+                    $*W.create_thunk: $ast.node,
+                    QAST::Op.new: :op<handle>,
+                      QAST::Op.new(:op<call>, $ast[1]), # Promised code block
+                      'CATCH',
+                        QAST::Op.new: :op<callmethod>,
+                          :name<handle-exception>,
+                          QAST::Op.new(:op<getcomp>,
+                            QAST::SVal.new: :value<perl6>),
+                          QAST::Op.new: :op<exception>
+                  );
+                return $ast;
+            }
             if !$ast.nosink && !$*COMPILING_CORE_SETTING && !%nosink{$ast.name} {
                 return $ast if $*ALREADY_ADDED_SINK_CALL;
                 $ast.sunk(1);
@@ -2736,12 +2751,13 @@ class Perl6::Actions is HLL::Actions does STDActions {
             $*W.install_lexical_magical($block, '$!');
         }
         make QAST::Op.new(
+            :node($/),
             :op('callmethod'),
             :name('start'),
             :returns($*W.find_symbol(['Promise'])),
             QAST::WVal.new( :value($*W.find_symbol(['Promise'])) ),
             $<blorst>.ast
-        );
+        ).annotate_self: 'promise_starter', 1;
     }
 
     method statement_prefix:sym<lazy>($/) {

src/core/traits.pm6

@@ -374,6 +374,12 @@ multi sub trait_mod:<of>(Routine:D $target, Mu:U $type) {
     $target.^mixin(Callable.^parameterize($type))
 }
 
+multi sub trait_mod:<is>(Code:D $r, :$hidden-from-backtrace!) {
+    $r.^mixin( role is-hidden-from-backtrace {
+        method is-hidden-from-backtrace(--> True) { }
+    }) if $hidden-from-backtrace;
+}
+
 multi sub trait_mod:<is>(Routine:D $r, :$hidden-from-backtrace!) {
     $r.^mixin( role is-hidden-from-backtrace {
         method is-hidden-from-backtrace(--> True) { }