Implement higher-level result handling in EVMC Rust bindings

[jakelang]

Part of #168.

[axic]

I think this should have two use cases:

1. Building a result object for data to be sent back from a VM execution
2. Acquiring a result from a call to be used by the VM execution

For case 1) the VM will have a status code, data, gas left (and if it is a create - an address). I suggest we have a constructor for this case.

For case 2) we have a raw evmc_result pointer which we need to consume by wrapping it into an EvmcResult. When dropping the EvmcResult in this case, the underlying release must be called.

[chfast]

If you may, prefix you commits with rust:

[axic]

Didn't we agree that the ExecutionResult instance has the data and address objects as Rust data structures (and not keep it boxed)?

[jakelang]

@axic Is this good for merge?

[chfast]

Add it to CHANGELOG if there is anything to mention. You can push new CHANGELOG entry to master if convenient.

[axic]

This code on its own is not that useful yet. Will add it to the changelog once we get #223 finished and merged.