Supply.split should only produce Str

The underlying Match objects only have meaning for the chunk that they
were created in.  There's no meaningful information that can be gathered
from them outside of that context.

src/core.c/Supply-factories.pm6

@@ -641,13 +641,13 @@
     # split the supply on the needle and adverbs
     multi method split(Supply:D: \needle) {
         supply {
-            my $str = "";  # can also be a Match object
+            my $str = "";  # prevent warning on first batch
             my @matches;
             whenever self -> \value {
                 done unless @matches = ($str ~ value).split(needle, |%_);
-                $str = @matches.pop;  # keep last for next batch
+                $str = @matches.pop.Str;  # keep last for next batch
 
-                emit $_ for @matches;
+                emit .Str for @matches;
 
                 LAST { emit $str }
             }