avoid warnings when processing `GasInt` for RLP (#712)

RLP does not support signed integer values, so we have to convert
`GasInt` to `uint64` when decoding / encoding via RLP.

eth/common/eth_types_rlp.nim

@@ -70,6 +70,18 @@ proc append*[T](w: var RlpWriter, val: Opt[T]) =
   else:
     w.append("")
 
+proc read*(rlp: var Rlp, T: type GasInt): T {.inline.} =
+  var res: uint64
+  rlp.read(res)
+  if res > GasInt.high.uint64:
+    raise (ref MalformedRlpError)(msg:
+      "GasInt value too large: " & $res)
+  res.GasInt
+
+proc append*(w: var RlpWriter, value: GasInt) =
+  doAssert value >= 0
+  w.append(value.uint64)
+
 proc appendTxLegacy(w: var RlpWriter, tx: Transaction) =
   w.startList(9)
   w.append(tx.nonce)