Simplify some Option / Result / ? operator patterns (#8653)

* Simplify some Option / Result / ? operator patterns

When those match a combinator exactly.

Tool-aided by [comby-rust](https://github.com/huitseeker/comby-rust).

* adjust after review

* adjust post-review

substrate/frame/democracy/src/lib.rs

@@ -1653,10 +1653,7 @@ impl<T: Config> Module<T> {
 		// To decode the enum variant we only need the first byte.
 		let mut buf = [0u8; 1];
 		let key = <Preimages<T>>::hashed_key_for(proposal_hash);
-		let bytes = match sp_io::storage::read(&key, &mut buf, 0) {
-			Some(bytes) => bytes,
-			None => return Err(Error::<T>::NotImminent.into()),
-		};
+		let bytes = sp_io::storage::read(&key, &mut buf, 0).ok_or_else(|| Error::<T>::NotImminent)?;
 		// The value may be smaller that 1 byte.
 		let mut input = &buf[0..buf.len().min(bytes as usize)];
 
@@ -1684,10 +1681,7 @@ impl<T: Config> Module<T> {
 		// * at most 5 bytes to decode a `Compact<u32>`
 		let mut buf = [0u8; 6];
 		let key = <Preimages<T>>::hashed_key_for(proposal_hash);
-		let bytes = match sp_io::storage::read(&key, &mut buf, 0) {
-			Some(bytes) => bytes,
-			None => return Err(Error::<T>::PreimageMissing.into()),
-		};
+		let bytes = sp_io::storage::read(&key, &mut buf, 0).ok_or_else(|| Error::<T>::PreimageMissing)?;
 		// The value may be smaller that 6 bytes.
 		let mut input = &buf[0..buf.len().min(bytes as usize)];
 
@@ -1761,10 +1755,7 @@ impl<T: Config> Module<T> {
 fn decode_compact_u32_at(key: &[u8]) -> Option<u32> {
 	// `Compact<u32>` takes at most 5 bytes.
 	let mut buf = [0u8; 5];
-	let bytes = match sp_io::storage::read(&key, &mut buf, 0) {
-		Some(bytes) => bytes,
-		None => return None,
-	};
+	let bytes = sp_io::storage::read(&key, &mut buf, 0)?;
 	// The value may be smaller than 5 bytes.
 	let mut input = &buf[0..buf.len().min(bytes as usize)];
 	match codec::Compact::<u32>::decode(&mut input) {