Function encode_storage_key_prefix 

pub fn encode_storage_key_prefix(
    pallet_name: &str,
    storage_entry: &str,
) -> [u8; 32]

Encode a storage key prefix from a pallet name and storage entry name. This prefix is the first 32 bytes of any storage key which comes from a pallet, and is essentially twox_128(pallet_name) + twox_128(storage_entry_name).