Make Externalities::ret consume self

[gavofyork]

Currently ret can be called more than once; it should consume self to avoid this. Will need refactoring so it's called from the owner (Executive, not JIT/Interpreter). This will make much of its code cleaner by avoiding dicking around with mut&s, where moving is actually what should be done.

[rphmeier]

I am interested in working on this.

[arkpar]

@rphmeier Cool, post here or reach us at https://gitter.im/ethcore/parity in case you need any help.