Transaction: Map `evmc_result` back to `Computation` result

This missing part of EVMC processing allows third-party EVMs to work.

It fixes EVMC result processing (at the top-level of calls, not nested calls)
to use the EVMC result object, instead of reading so much internal state of the
Nimbus `Computation` object.

It has been tested by calling [`evmone`](https://github.com/ethereum/evmone)
and getting useful results with tracing enabled (`showTxCalls = true`).  It's
even able to run parts of the fixtures test suite.

There are other issues with account balances, etc that need to be worked on to
get the correct _final_ results, but the EVM execution is correct with this.

Signed-off-by: Jamie Lokier <jamie@shareable.org>

nimbus/transaction/call_common.nim

@@ -174,6 +174,28 @@ proc setupHost(call: CallParams): TransactionHost =
 
   return host
 
+when defined(evmc_enabled):
+  import ./host_services
+  proc doExecEvmc(host: TransactionHost, call: CallParams) =
+    var callResult = evmcExecComputation(host)
+    let c = host.computation
+
+    if callResult.status_code == EVMC_SUCCESS:
+      c.error = nil
+    elif callResult.status_code == EVMC_REVERT:
+      c.setError("EVMC_REVERT", false)
+    else:
+      c.setError($callResult.status_code, true)
+
+    c.gasMeter.gasRemaining = callResult.gas_left
+    c.msg.contractAddress = callResult.create_address.fromEvmc
+    c.output = if callResult.output_size <= 0: @[]
+               else: @(makeOpenArray(callResult.output_data,
+                                     callResult.output_size.int))
+    if not callResult.release.isNil:
+      {.gcsafe.}:
+        callResult.release(callResult)
+
 proc runComputation*(call: CallParams): CallResult =
   let host = setupHost(call)
   let c = host.computation
@@ -188,7 +210,7 @@ proc runComputation*(call: CallParams): CallResult =
       db.subBalance(call.sender, call.gasLimit.u256 * call.gasPrice.u256)
 
   when defined(evmc_enabled):
-    discard evmcExecComputation(host)
+    doExecEvmc(host, call)
   else:
     execComputation(host.computation)