Have Ext::ret take self by value (#1187)

* refactor externalities::ret to take self by-value, add GasLeft enum, and alter evm::Result.

* remove unused imports, StopExecutionWithGasLeft variant

* adjust tests

* remove extraneous call to reserve

* update json_tests Ext to match new trait

* adjust executive json_test

* have evms own their memory for their entire lifetime

* make finalize API more friendly

* indentation fix

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