Test Nimbus EVMC implementation against other EVMC implementation

[jangko]

In https://github.com/status-im/nim-evmc, we have the interface of EVMC.

This issue related to or an addition to #445 and #114.

In this repo we have completed the implementation of the so called 'EVMC Host' while the implementation of 'EVMC VM' still ongoing.

This 'EVMC Host' already tested against Nimbus EVM, but we still need to test it against other implementation and make sure our implementation is correct.
we can test our EVMC Host against:

• evmone
• aleth

If Nimbus 'EVMC VM' already completed, we can also test it against:

• aleth

the choices are not much because they either incomplete or using outdated ABI version.
Nimbus implement EVMC ABI ver 7.0

[jangko]

Turn out aleth is a bit too much for us. It's gargantuan size with a lot of dependencies takes a lot of time to download and build.
Alternative option: we test Nimbus 'EVMC Host' against evmone only, and then use this 'EVMC Host' to test Nimbus 'EVMC VM'.

We can add a new CI job to Travis CI matrix dedicated for this test.

Nimbus have it's own 'Native Host' API separated from 'EVMC Host' by evmc_enabled switch.