contracts: Don't read the previous value when overwriting a storage item (#7879)

* Add `len` function that can return the length of a storage item efficiently

* Make use of the new len function in contracts

* Fix benchmarks

* cargo run --release --features=runtime-benchmarks --manifest-path=bin/node/cli/Cargo.toml -- benchmark --chain=dev --steps=50 --repeat=20 --pallet=pallet_contracts --extrinsic=* --execution=wasm --wasm-execution=compiled --heap-pages=4096 --output=./frame/contracts/src/weights.rs --template=./.maintain/frame-weight-template.hbs

* Remove unused imports

Co-authored-by: Parity Benchmarking Bot <admin@parity.io>

substrate/frame/support/src/storage/child.rs

@@ -243,3 +243,21 @@ pub fn root(
 		),
 	}
 }
+
+/// Return the length in bytes of the value without reading it. `None` if it does not exist.
+pub fn len(
+	child_info: &ChildInfo,
+	key: &[u8],
+) -> Option<u32> {
+	match child_info.child_type() {
+		ChildType::ParentKeyId => {
+			let mut buffer = [0; 0];
+			sp_io::default_child_storage::read(
+				child_info.storage_key(),
+				key,
+				&mut buffer,
+				0,
+			)
+		}
+	}
+}