Add eos-vm to wasm-engines

[axic]

It has a benchmarking tool: https://github.com/EOSIO/eos-vm/blob/master/tools/bench_interp.cpp

Need to make sure to disable metering as even the watchdog could be implemented in a way detrimental to speed.

@poemm has run some tests and his findings/patches can be found here: https://gist.github.com/poemm/cec9e8b3e085651dab6ec06bc5dd9b0e

[axic]

It seems the above tool has "metering" disabled because it uses null_watchdog(), see more about that here: https://github.com/EOSIO/eos-vm/blob/ee011884397d80d787868c70995200c0c1b19f13/docs/OVERVIEW.md