simplify EVM callback (#2282)

The EVM callbacks currently don't require closures, so we can use
`nimcall` to reduce overhead slightly.

nimbus/evm/interpreter/op_handlers/oph_defs.nim

@@ -1,5 +1,5 @@
 # Nimbus
-# Copyright (c) 2018-2023 Status Research & Development GmbH
+# Copyright (c) 2018-2024 Status Research & Development GmbH
 # Licensed under either of
 # * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
 # http:https://www.apache.org/licenses/LICENSE-2.0)
@@ -20,11 +20,10 @@ import
 type
  Vm2Ctx* = tuple
  cpt: Computation ## computation text
- rc: int ## return code from op handler
 
  Vm2OpFn* = ## general op handler, return codes are passed
  ## back via argument descriptor ``k``
- proc(k: var Vm2Ctx) {.gcsafe, raises: [CatchableError].}
+ proc(k: var Vm2Ctx) {.nimcall, gcsafe, raises: [CatchableError].}
 
 
  Vm2OpHanders* = tuple ## three step op code execution, typically