Fix memory leak; add simple binary message data

Note that this changes the API slightly - $msg.data() now returns a Buf,
while $msg.data-str() now returns a string.

This patch also uses the new DESTROY method, which will limit this
module to the backends that support it (moarvm only IIRC).
Merge pull request #2 from retupmoca/master

Fix memory leak; add simple binary message data