The call tracer and prestate tracer store data JSON-encoded in memory. In order to support alternative encodings (specifically RLP), it's better to keep data a native format during tracing. This PR does marshalling at the end, using gencodec.

OBS! 
This PR changes the call tracer result slightly:

-  Order of type and value fields are changed (should not matter). 
-  Output fields are completely omitted when they're empty (no more output: "0x"). Previously, this was only _sometimes_ omitted (e.g. when call ended in a non-revert error) and otherwise 0x when the output was actually empty.