Fix encoding of nested static tuples in dynamic tuple (#220)

Fixed #202

This PR fixes the "head length" calculation for nested static tuples. This would cause offsets of dynamic data (such as dynamic arrays) to be incorrectly computed in certain cases (such as encoding a dynamic array within a tuple including nested static tuples). If you consider data of:
```
Tuple( // dynamic
  Tuple( // static
    Tuple(
      Bool,
      U256,
    ),
  Array(U256),
)
```

The static tuple was incorrectly computing its "head length" to be 32 instead of 64 - in other words, nested static tuples weren't correctly being flattened. This caused the encoded offset of the dynamic array to be off by 32 bytes, and cause decoding failure described in the linked case.

### Test Plan

Added a new unit test to verify encoding is correct.

We can use Ethers.js as a reference implementation:
```
> ethers.utils.defaultAbiCoder.encode(["(((bool,uint256)),uint256[])"], [[[[false, 0x777]],[0x42,0x1337]]])
0x
0000000000000000000000000000000000000000000000000000000000000020 // offset to start of dynamic tuple data
0000000000000000000000000000000000000000000000000000000000000000 // nested static tuple bool value
0000000000000000000000000000000000000000000000000000000000000777 // nested static tuple uint value
0000000000000000000000000000000000000000000000000000000000000060 // offset to start of dynamic array *within* dynamic tuple "tail"
0000000000000000000000000000000000000000000000000000000000000002 // dynamic array length
0000000000000000000000000000000000000000000000000000000000000042 // first array value
0000000000000000000000000000000000000000000000000000000000001337 // second array value
```

ethabi/src/encoder.rs

@@ -52,7 +52,7 @@ impl Mediate {
 	fn head_len(&self) -> u32 {
 		match *self {
 			Mediate::Raw(ref raw) => 32 * raw.len() as u32,
-			Mediate::RawTuple(ref mediates) => 32 * mediates.len() as u32,
+			Mediate::RawTuple(ref mediates) => mediates.iter().map(|mediate| mediate.head_len()).sum(),
 			Mediate::Prefixed(_)
 			| Mediate::PrefixedArray(_)
 			| Mediate::PrefixedArrayWithLength(_)
@@ -812,4 +812,29 @@ mod tests {
 		.to_vec();
 		assert_eq!(encoded, expected);
 	}
+
+	#[test]
+	fn encode_dynamic_tuple_with_nested_static_tuples() {
+		let token = {
+			use crate::Token::*;
+			Tuple(vec![
+				Tuple(vec![Tuple(vec![Bool(false), Uint(0x777.into())])]),
+				Array(vec![Uint(0x42.into()), Uint(0x1337.into())]),
+			])
+		};
+		let encoded = encode(&[token]);
+		let expected = hex!(
+			"
+			0000000000000000000000000000000000000000000000000000000000000020
+			0000000000000000000000000000000000000000000000000000000000000000
+			0000000000000000000000000000000000000000000000000000000000000777
+			0000000000000000000000000000000000000000000000000000000000000060
+			0000000000000000000000000000000000000000000000000000000000000002
+			0000000000000000000000000000000000000000000000000000000000000042
+			0000000000000000000000000000000000000000000000000000000000001337
+		"
+		)
+		.to_vec();
+		assert_eq!(encoded, expected);
+	}
 }