1. Send all EVM executions through the EVMC `execute` function.
It leads to the same place in the end as calling `Computation` before, but
`execute` is the API function used by all EVMC implementations, and it is
very explicit what data is passed back and forth.
2. As a consequence this starts using the new `host_services` code from EVM, so
this is a significant change to the paths used for account state processing.
3. Because we will have to remove the `newComputation` call on the host side,
anticipating that the contract code is now saved in `host` instead of being
copied around. As it's saved in `host`, there is no need to pass it
separately to `evmcExecComputation`.
Signed-off-by: Jamie Lokier <jamie@shareable.org>