Two-phase initialization

[chfast]

We should split the evmc_context (host-side execution context) from evmc_context_fn_table. The evmc_context_fn_table should be renamed to evmc_host_interface and passed to the VM instance constructor. That would require splitting the ABI check from the VM instance creation.

[axic]

@chfast wasn't this done by #427?

[chfast]

and passed to the VM instance constructor. That would require splitting the ABI check from the VM instance creation.

^^^ The second part is not done yet.