Computed jump in VM

[ypiguet-epfl]

The current VM seems to support the following feature: one can jump to a stored or computed address by pushing a displacement onto the stack and returning from it (ASEBA_BYTECODE_SUB_RET).

To allow third-party compilers to benefit from it to do things like function pointers, virtual methods, or multitasking using an alternative return stack (the stack pointer is unfortunately reset in function AsebaVMSetupEvent), I request a firm commitment to keep this capability in future versions of the VM and in tools involved with bytecode (i.e. keep a single stack for data and return addresses and don't reject code using it), or to offer an alternative way offering equivalent functionality.

See #777 for the reasons I'm making this request.