Document panicking instructions

[ctiedt]

The current specification does not state if instructions may panic. Off the top of my head, here are instructions for which this should be clarified:

• panic - self-explanatory.
• syscall - I'm not sure any current soil implementation uses panicking syscalls, but they could be useful for error handling. Also, invalid syscall numbers may cause a panic.
• memory instructions - Out-of-bounds accesses are specified to cause panics. However, it might be helpful to annotate the memory instructions as panicking.
• Division (div and fdiv) - Divide by zero causes a panic in all implementations that I am aware of, but the specification does not state this.
• tryend, ret - If these instruction appear without a trystart or call respectively before, they should likely panic.
• jump, cjump, call - jumps to illegal addresses should panic.

There is also the question of handling instructions that specify invalid registers. However, since the set of valid registers is well-defined, bytecode containing invalid registers should be considered as malformed or invalid from the start. We might specify that a VM should refuse to execute such code at all or immediately end execution without the possibility of error recovery.