Make error on "my $x is Int" more awesomeo

  $ 6 'my $x is Int'
  ===SORRY!=== Error while compiling -e
  is trait on $-sigil variable not yet implemented. Sorry.
  Did you mean: my Int $x?

- as recently discussed on perl6-language
- added optional "did-you-mean" parameter to X::NYI
- used that in $*W.container_type_info

src/Perl6/World.nqp

@@ -1682,7 +1682,10 @@ class Perl6::World is HLL::World {
         }
         else {
             if @cont_type {
-                self.throw($/, 'X::NYI', :feature('is trait on $-sigil variable'));
+                self.throw($/, 'X::NYI',
+                  :feature('is trait on $-sigil variable'),
+                  :did-you-mean("my {@cont_type[0].HOW.name(@cont_type[0])} $sigil{~$<variable><desigilname>}")
+                );
             }
             %info<container_base>     := self.find_symbol(['Scalar']);
             %info<container_type>     := %info<container_base>;

src/core/Exception.pm

@@ -748,7 +748,12 @@ my role X::Pod                 { }
 
 my class X::NYI is Exception {
     has $.feature;
-    method message() { "$.feature not yet implemented. Sorry. " }
+    has $.did-you-mean;
+    method message() {
+        my $msg = "$.feature not yet implemented. Sorry.";
+        $msg ~= "\nDid you mean: {$.did-you-mean.gist}?" if $.did-you-mean;
+        $msg
+    }
 }
 my class X::Comp::NYI is X::NYI does X::Comp { };
 my class X::NYI::Available is X::NYI {