Externalize suggestions and tips

The X::Method::NotFound error is hard to test for.  This commit
adds the "suggestions" and "tips" method, so that one can test for
(not) expected suggestions, and whether any tips were generated.

Also only use numeric weights in suggestions: there was a mix of
strings and integers, that can't be right  :-)

src/core.c/Exception.pm6

@@ -153,6 +153,10 @@ my class X::Method::NotFound is Exception {
     has $.typename;
     has Bool $.private;
     has $.addendum;
+    has $.message is built(False);
+    has @.suggestions is built(False);
+    has @.tips is built(False);
+
     # This attribute is an implementation detail. Not to be documented.
     has $.in-class-call;
 
@@ -162,14 +166,13 @@ my class X::Method::NotFound is Exception {
           !! "of type '$.typename'"
     }
 
-    method message() {
+    method TWEAK() {
         my @message = $.private
           ?? "No such private method '!$.method' for invocant $.of-type"
           !! "No such method '$.method' for invocant $.of-type";
 
         @message.push: $.addendum if $.addendum;
 
-        my @tips;
         my $indirect-method = $.method.starts-with("!")
                                 ?? $.method.substr(1)
                                 !! "";
@@ -204,7 +207,7 @@ my class X::Method::NotFound is Exception {
         sub find_public_suggestion($before, $after --> Nil) {
             if $before.fc eq $after.fc {
                 $public_suggested = 1;
-                %suggestions{$after} = "";  # assume identity
+                %suggestions{$after} = 0;  # assume identity
             }
             else {
                 my $dist := StrDistance.new(
@@ -213,7 +216,7 @@ my class X::Method::NotFound is Exception {
                 );
                 if $dist <= $max_length {
                     $public_suggested = 1;
-                    %suggestions{$after} = ~$dist;
+                    %suggestions{$after} = $dist;
                 }
             }
         }
@@ -248,41 +251,43 @@ my class X::Method::NotFound is Exception {
                 );
                 if $dist <= $max_length {
                     $private_suggested = 1;
-                    %suggestions{"!$method_name"} = ~$dist
+                    %suggestions{"!$method_name"} = $dist
                         unless $indirect-method eq $method_name;
                 }
             }
         }
 
         if $indirect-method && !$.private && $private_suggested {
-            @tips.push: "Method name starts with '!', did you mean 'self!\"$indirect-method\"()'?";
+            @!tips.push: "Method name starts with '!', did you mean 'self!\"$indirect-method\"()'?";
         }
 
-        if +%suggestions == 1 {
-            @tips.push: "Did you mean '%suggestions.keys()'?";
+        @!suggestions = %suggestions.sort(*.value).map(*.key).head(4);
+
+        if @!suggestions == 1 {
+            @!tips.push: "Did you mean '@!suggestions[0]'?";
         }
-        elsif +%suggestions > 1 {
-            @tips.push: "Did you mean any of these: { %suggestions.sort(*.value)>>.key.head(4).map( { "'$_'" } ).join(", ") }?";
+        elsif @!suggestions {
+            @!tips.push: "Did you mean any of these: { @!suggestions.map( { "'$_'" } ).join(", ") }?";
         }
 
         if !$indirect-method
            &&($private_suggested ^^ $public_suggested)
            && ($private_suggested ^^ $.private)
         {
-            @tips.push: "Perhaps a " ~ ($private_suggested ?? "private" !! "public") ~ " method call must be used."
+            @!tips.push: "Perhaps a " ~ ($private_suggested ?? "private" !! "public") ~ " method call must be used."
         }
 
-        if +@tips > 1 {
-            @tips = @tips.map: "\n" ~ ("- " ~ *).naive-word-wrapper(:indent("    "));
+        if @!tips > 1 {
+            @!tips = @!tips.map: "\n" ~ ("- " ~ *).naive-word-wrapper(:indent("    "));
             @message.push: ($.addendum ?? "Other possible" !! "Possible") ~ " causes are:";
         }
-        elsif @tips {
-            @message.push: @tips.shift;
+        elsif @!tips {
+            @message.push: @!tips.shift;
         }
 
-        @message[0] ~= "." if +@message > 1;
+        @message[0] ~= "." if @message > 1;
 
-        @message.join(" ").naive-word-wrapper ~ @tips.join
+        $!message = @message.join(" ").naive-word-wrapper ~ @!tips.join
     }
 }