Finalise the bytecode interpreter

[kontheocharis]

• 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.