implement 'chainId' opcode

nimbus/vm/interpreter/opcode_values.nim

@@ -79,7 +79,7 @@ fill_enum_holes:
       Difficulty =     0x44, # Get the block's difficulty.
       GasLimit =       0x45, # Get the block's gas limit.
 
-      ChainID =        0x46, # Get current chain’s EIP-155 unique identifier.
+      ChainId =        0x46, # Get current chain’s EIP-155 unique identifier.
       SelfBalance =    0x47, # Get current contract's balance.
 
       # 50s: Stack, Memory, Storage and Flow Operations

nimbus/vm/interpreter/opcodes_impl.nim

@@ -372,10 +372,10 @@ op gasLimit, inline = true:
   ## 0x45, Get the block's gas limit
   push: computation.vmState.gasLimit
 
-op chainID, inline = true:
+op chainId, inline = true:
   ## 0x46, Get current chain’s EIP-155 unique identifier.
   # TODO: this is a stub
-  push: 0
+  push: computation.vmState.chaindb.config.chainId
 
 op selfBalance, inline = true:
   ## 0x47, Get current contract's balance.

nimbus/vm/interpreter_dispatch.nim

@@ -217,7 +217,7 @@ let ConstantinopleOpDispatch {.compileTime.}: array[Op, NimNode] = genConstantin
 
 proc genIstanbulJumpTable(ops: array[Op, NimNode]): array[Op, NimNode] {.compileTime.} =
   result = ops
-  result[ChainID] = newIdentNode "chainID"
+  result[ChainId] = newIdentNode "chainId"
   result[SelfBalance] = newIdentNode "selfBalance"
 
 let IstanbulOpDispatch {.compileTime.}: array[Op, NimNode] = genIstanbulJumpTable(ConstantinopleOpDispatch)