Implement a fork-based backend as stepping stone to distributing across machines

[lukstafi]

Parany. Will require accordingly refactoring both Session and Exec_as_gccjit.

[lukstafi]

Not helpful, Domain-based devices already implemented.

[lukstafi]

Restoring this as it provides more angles on the design space. (And even possibility for efficiency gain due to not using domain-based GC, but that should be irrelevant to the decision.)