Refactor slurp($filename) to use Parrot's .readall($filename);

this results in a massive speedup for slurping a file into a string.
See TT #1749 for related parrot details.

src/core/IO.pm

@@ -214,10 +214,13 @@ sub close($handle) {
 }
 
 sub slurp($filename) {
-    my $handle = open($filename, :r);
-    my $contents = $handle.slurp();
-    $handle.close();
-    $contents
+    ## Although it's tempting to delegate to IO.slurp above, Parrot
+    ## currently suffers a serious (25x) performance degradation when 
+    ## using readall() on an already-opened FileHandle.  Much faster
+    ## is to use readall($filename), which we do here. See TT #1749.
+    my $PIO = Q:PIR { %r = root_new['parrot';'FileHandle'] };
+    $PIO.encoding('utf8');
+    $PIO.readall($filename);
 }
 
 sub unlink($filename) {