Implement serialization for [u16; 32], DRYing it with [u8; *]

In the next commit we'll need serialization for `[u16; 32]`, which
we add here, unifying it with the `[u8; *]` serialization macro.

Author: TheBlueMatt

lightning/src/util/ser.rs

@@ -586,28 +586,34 @@ impl_array!(64); // for ecdsa::Signature and schnorr::Signature
 impl_array!(66); // for MuSig2 nonces
 impl_array!(1300); // for OnionPacket.hop_data
 
-impl Writeable for [u16; 8] {
-	#[inline]
-	fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
-		for v in self.iter() {
-			w.write_all(&v.to_be_bytes())?
+macro_rules! impl_u16_array {
+	($len: expr) => {
+		impl Writeable for [u16; $len] {
+			#[inline]
+			fn write<W: Writer>(&self, w: &mut W) -> Result<(), io::Error> {
+				for v in self.iter() {
+					w.write_all(&v.to_be_bytes())?
+				}
+				Ok(())
+			}
 		}
-		Ok(())
-	}
-}
 
-impl Readable for [u16; 8] {
-	#[inline]
-	fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
-		let mut buf = [0u8; 16];
-		r.read_exact(&mut buf)?;
-		let mut res = [0u16; 8];
-		for (idx, v) in res.iter_mut().enumerate() {
-			*v = (buf[idx*2] as u16) << 8 | (buf[idx*2 + 1] as u16)
+		impl Readable for [u16; $len] {
+			#[inline]
+			fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
+				let mut buf = [0u8; $len*2];
+				r.read_exact(&mut buf)?;
+				let mut res = [0u16; $len];
+				for (idx, v) in res.iter_mut().enumerate() {
+					*v = (buf[idx*2] as u16) << 8 | (buf[idx*2 + 1] as u16)
+				}
+				Ok(res)
+			}
 		}
-		Ok(res)
 	}
 }
+impl_u16_array!(8);
+impl_u16_array!(32);
 
 /// A type for variable-length values within TLV record where the length is encoded as part of the record.
 /// Used to prevent encoding the length twice.