Add snapshots

[rmgaray]

Overview

This PR adds snapshots to the execution of all MeTTa programs. It brings a set of changes to how MeTTa programs are run from Rholang as well.

• MeTTa programs must now have a main entrypoint which takes one parameter. This parameter is currently set to dummy, so it should not be inspected by the MeTTa program.
• All MeTTa programs are now executed inside a PeTTa session, which is located under the petta_sessions directory. The name of the session is the Blake2B-256 hash of the program's contents.
• MeTTa programs provided from Rholang are automatically injected with snapshots at every X inferences (see the implementation of petta_lib_snapshot).
• Session folders live only as much as necessary: if a MeTTa program succeeds or fails in any expected way (i.e: by producing some IntepreterError) the session is promptly cleaned. They will also be cleaned if the f1r3node panics, which may be desirable or not.