Add safety information about the Readable trait.

[Razican]

Currently, in the VM Readable trait we have a # Safety section but no safety information. Let's add it.

[HalidOdat]

It's unsafe because it should only be implemented on type that don't implement Drop, since primitives like u32, f64 don't implement Drop we can "safely" implement unsafe Readable. Currently rust stable does not support negative trait bounds we could call drop on a type twice, or more or not at all. We could potentially go around this with ManuallyDrop, but then we have to handle destruction somewhere else. In any case we currently don't need to have objects that implement Drop as opcode operand.