Problem: curve_encoding_t involves unnecesary heap allocations and copying of message data

Solution: use crypto_box_easy_afternm and crypto_box_open_easy_afternm
in-place

src/msg.hpp

@@ -161,6 +161,8 @@ class msg_t
     //  references drops to 0, the message is closed and false is returned.
     bool rm_refs (int refs_);
 
+    void shrink (size_t new_size_);
+
     //  Size in bytes of the largest message that is still copied around
     //  rather than being reference-counted.
     enum