EVMC: Use the same host interface for nested calls as top-level

Prior to this patch, top-level EVM executions and nested EVM executions did
their `getStorage` and other requests using a completely different set of host
functions.  It was just unfinished, to get top-level "new" EVMC working.

This finishes the job - it stops using the old methods.  Effect:

- Functionality added at the EVMC host level will be used by all EVM calls.
  (The target here is Beam Sync).

- The old set of functions are no longer used, so they can be removed.

- When EVMC host call tracing is enabled (`showTxCalls = true`), it traces
  the calls from nested EVM executions as well as top-level.

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

nimbus/transaction/evmc_host_glue.nim

@@ -108,6 +108,7 @@ proc evmcExecComputation*(host: TransactionHost): EvmcResult {.inline.} =
     return
 
   let hostContext = cast[evmc_host_context](host)
+  host.hostInterface = hostInterface.unsafeAddr
 
   # Without `{.gcsafe.}:` here, the call via `vm.execute` results in a Nim
   # compile-time error in a far away function.  Starting here, a cascade of

nimbus/transaction/host_call_nested.nim

@@ -112,16 +112,25 @@ proc beforeExecEvmcNested(host: TransactionHost, msg: EvmcMessage): Computation
     # contribute to the stack frame of `callEvmcNested` below.
     {.noinline.} =
   host.showCallEntry(msg)
-  if msg.kind == EVMC_CREATE or msg.kind == EVMC_CREATE2:
-    return beforeExecCreateEvmcNested(host, msg)
-  else:
-    return beforeExecCallEvmcNested(host, msg)
+  let c = if msg.kind == EVMC_CREATE or msg.kind == EVMC_CREATE2:
+            beforeExecCreateEvmcNested(host, msg)
+          else:
+            beforeExecCallEvmcNested(host, msg)
+  when defined(evmc_enabled):
+    c.host.init(cast[ptr nimbus_host_interface](host.hostInterface),
+                cast[typeof(c.host.context)](host))
+  host.saveComputation.add(host.computation)
+  host.computation = c
+  return c
 
 proc afterExecEvmcNested(host: TransactionHost, child: Computation,
                          kind: EvmcCallKind): EvmcResult
     # This function must be declared with `{.noinline.}` to make sure it doesn't
     # contribute to the stack frame of `callEvmcNested` below.
     {.noinline.} =
+  host.computation = host.saveComputation[^1]
+  host.saveComputation[^1] = nil
+  host.saveComputation.setLen(host.saveComputation.len - 1)
   if kind == EVMC_CREATE or kind == EVMC_CREATE2:
     afterExecCreateEvmcNested(host, child, result)
   else:

nimbus/transaction/host_types.nim

@@ -63,6 +63,8 @@ type
     touchedAccounts*: HashSet[EthAddress]
     selfDestructs*:   HashSet[EthAddress]
     depth*:           int
+    saveComputation*: seq[Computation]
+    hostInterface*:   ptr evmc_host_interface
 
 # These versions of `toEvmc` and `fromEvmc` don't flip big/little-endian like
 # the older functions in `evmc_helpers`.  New code only flips with _explicit_