Fix bad assumption about methods being closures.

They're not, since they're looked up via a table in the meta-object.
Thus this could cause data races when paths were being manipulated
from different threads.

src/core/IO/Path.pm

@@ -24,13 +24,13 @@ my class IO::Path is Cool does IO {
     }
 
     method !new-from-absolute-path($path, :$SPEC = $*SPEC, Str() :$CWD = $*CWD) {
-        method !set-absolute() {
-            $!is-absolute = True;
-            $!abspath := $path;
-            self;
-        }
+        self.bless(:$path, :$SPEC, :$CWD)!set-absolute($path);
+    }
 
-        self.bless(:$path, :$SPEC, :$CWD)!set-absolute;
+    method !set-absolute($path) {
+        $!is-absolute = True;
+        $!abspath := $path;
+        self;
     }
 
     proto method new(|) {*}