Make searching for method name suggestions a bit more resilient

Some bugs, especially when dealing with the setting and additions,
can cause strange types of values to occur.  Make sure that StrDistance
only gets strings, and that the suggestion is stored as a string.
This should have made searching for the "is_built" problem a *lot*
easier.

src/core.c/Exception.pm6

@@ -185,9 +185,9 @@ my class X::Method::NotFound is Exception {
 
         if nqp::can($!invocant.HOW, 'methods') {
             for $!invocant.^methods(:all)>>.name -> $method_name {
-                my $dist = StrDistance.new(:before($.method), :after($method_name));
+                my $dist = StrDistance.new(:before($.method), :after(~$method_name));
                 if $dist <= $max_length {
-                    %suggestions{$method_name} = $dist;
+                    %suggestions{$method_name} = ~$dist;
                 }
             }
         }