* 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

[ci skip]

* Groundwork for basic VM tracing.

* RPC endpoint for VM tracing and ser/de types ready.

* Create VMTracer trait.

* Rearchitected VM tracing to reflect existing tracing.

Should more or less work now.

* Integrated VM tracing into JSONRPC.

* Fix ethcore module tests.

* Add tests for VM tracing.

* Fix consensus test code.

* Fix mock tests.

* Added VM trace information for post-execution stuff.

* Fix max-value calls and add "creates" field to getTransaction.

* Tests for VM tracing.

* Don't implement the trait with unimplemented.

* Remove invlaid comment.

* Fix tests.