Finalise the bytecode interpreter

- Change the format to instructions without data - as in, we only use the instruction as a discriminant for the number of bytes that follow that instruction for reading. This is needed for instructions that potentially deal with large integer values, e.g. `i64`, `u128`, and so that they don't take up more space than is required of them.

- Implement syscalls in a generic way, the VM should be able to use a `syscall` instruction that can take operands, and then write the result of the operand into the specified location.

- Implement an interface that supports reference-counted memory management within the VM, i.e. things that are declared to be reference counted use a wrapper within the VM to perform the reference counts.