fixes related to EIP-4399/EIP-3675

- fix `RANDOM` opcode export
- fix `difficulty` return value in VMState

nimbus/vm/interpreter.nim

@@ -12,7 +12,7 @@
 import
   ./interpreter/opcode_values as vmo
 export
-  vmo.Op
+  vmo.Op, vmo.Random
 
 
 # see vm_message

nimbus/vm/state.nim

@@ -308,6 +308,10 @@ method blockNumber*(vmState: BaseVMState): BlockNumber {.base, gcsafe.} =
   vmState.parent.blockNumber + 1
 
 method difficulty*(vmState: BaseVMState): UInt256 {.base, gcsafe.} =
+  if vmState.fork >= FkPostMerge:
+    # EIP-4399/EIP-3675
+    0.u256
+  else:
     vmState.chainDB.config.calcDifficulty(vmState.timestamp, vmState.parent)
 
 method baseFee*(vmState: BaseVMState): UInt256 {.base, gcsafe.} =

nimbus/vm2/state.nim

@@ -295,6 +295,10 @@ method blockNumber*(vmState: BaseVMState): BlockNumber {.base, gcsafe.} =
   vmState.parent.blockNumber + 1
 
 method difficulty*(vmState: BaseVMState): UInt256 {.base, gcsafe.} =
+  if vmState.fork >= FkPostMerge:
+    # EIP-4399/EIP-3675
+    0.u256
+  else:
     vmState.chainDB.config.calcDifficulty(vmState.timestamp, vmState.parent)
 
 method baseFee*(vmState: BaseVMState): UInt256 {.base, gcsafe.} =

nimbus/vm_internals.nim

@@ -133,6 +133,7 @@ else:
     eGmt.refundGas,
     eGmt.returnGas,
     fVmo.Op,
+    fVmo.Random,
     gVmg.isCreate,
     hStk.Stack,
     hStk.`$`,