Cleaned and fixed actors and actors_prim.

The C and Grace AID types are now Copyable (so they can be sent via
message passing).

Added "Signal" type and class. Used to define convenient primitives like
STOP and KILL. They will be used in the "extended" actors library.
Added actors_lib.

Contains handy classes and types for more "OO-style" actors. These
actors automatically handle signals and factor out the receive loop. See
sample/actors/pingpong_oo.grace for an example.