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>