Check node declaration wrt safety and memory.

  More precisely, a node that has memory ougth to be declared using
  "node", and using "function" otherwise.

  Moreover, a node that performs side-effects (i.e., if it calls an
  extern node that performs side effects) ougth to be declared as
  "unsafe".

  Safe/unsafe mismatches raise an error.

  Memory mismatches raise an error in one way (a "function" that uses
  memory), and a warning in the other way (a "node" that uses no
  memory).

Also fix some errors done when transmitting the unsafe flag (well, it's
the first time I use it!).