Fix ABI encoding of Ethereum Addresses

They should be encoded as if they were UInt160, which means they
should be left-padded with zeroes, instead of right-padded.

nitro/abi.nim

@@ -1,5 +1,6 @@
 import pkg/stew/endians2
 import pkg/stint
+import ./types
 
 type
   Abi* = object
@@ -61,6 +62,9 @@ proc write*(writer: var AbiWriter, bytes: seq[byte]) =
   else:
     writer.writeLater(bytes)
 
+proc write*(writer: var AbiWriter, address: EthAddress) =
+  writer.padleft(address.toArray)
+
 proc startTuple*(writer: var AbiWriter) =
   writer.tuples.add(Tuple())
 

nitro/types.nim

@@ -5,4 +5,9 @@ export stint
 
 type
   UInt48* = range[0'u64..2'u64^48-1]
-  EthAddress* = array[20, byte]
+  EthAddress* = distinct array[20, byte]
+
+proc toArray*(address: EthAddress): array[20, byte] =
+  array[20, byte](address)
+
+proc `==`*(a, b: EthAddress): bool {.borrow.}

tests/nitro/examples.nim

@@ -24,6 +24,9 @@ proc example*(_: type UInt256): UInt256 =
 proc example*(_: type UInt128): UInt128 =
   UInt128.fromBytes(array[16, byte].example)
 
+proc example*(_: type EthAddress): EthAddress =
+  EthAddress(array[20, byte].example)
+
 proc example*(_: type Channel): Channel =
   Channel(
     nonce: UInt48.example,

tests/nitro/testAbi.nim

@@ -1,5 +1,6 @@
 import std/unittest
 import pkg/nitro/abi
+import pkg/nitro/types
 import pkg/stint
 import ./examples
 
@@ -61,6 +62,10 @@ suite "ABI encoding":
     let bytes33len = Abi.encode(bytes33.len.uint64)
     check Abi.encode(bytes33) == bytes33len & bytes33 & 31.zeroes
 
+  test "encodes ethereum addresses":
+    let address = EthAddress.example
+    check Abi.encode(address) == 12.zeroes & @(address.toArray)
+
   test "encodes tuples":
     let a = true
     let b = @[1'u8, 2'u8, 3'u8]