Allow `SovState` use Borrow to behave similar to std::collections::HashMap

[citizen-stig]

Consider following rust code, which compares usage of HashMap and StateMap:

use std::collections::HashMap;
use sov_state::{DefaultStorageSpec, Prefix, ProverStorage, StateMap, WorkingSet};

pub fn hashmap() {
    let mut map: HashMap<Vec<u8>, u64> = HashMap::new();

    map.insert(vec![1, 2, 3], 1);
    map.insert(vec![4, 5, 6], 2);
    map.insert(vec![7, 8, 9], 3);

    println!("HASH MAP: {:?}", map);

    let check_this: [u8; 3] = [1, 2, 3];

    let v = map.get(&check_this[..]);
    assert!(v.is_some());
    assert_eq!(v, Some(&1));
}

pub fn sovstatemap() {
    let tmpdir = tempfile::tempdir().unwrap();
    let mut working_set = WorkingSet::new(ProverStorage::<DefaultStorageSpec>::with_path(tmpdir.path()).unwrap());


    let map: StateMap<Vec<u8>, u64> = StateMap::new(Prefix::new(vec![1, 2, 3]));

    let a = vec![1, 2, 3];
    let b = vec![4, 5, 6];
    let c = vec![7, 8, 9];
    map.set(&a, &1, &mut working_set);
    map.set(&b, &2, &mut working_set);
    map.set(&c, &3, &mut working_set);

    // Uncomment this to get compiler error
    // let check_this: [u8; 3] = [1, 2, 3];
    // let v = map.get(&check_this[..]);
    // assert!(v.is_some());

}

It would be nice to be able to use slice the same as with standard HashMap, it will remove some unnecessary clonings in default STF.