Make `Store::insert` return the previous value

[Thibault Martinez]

https://docs.rs/iota_stronghold/0.5.1/iota_stronghold/types/struct.Store.html#method.insert

It is pretty standard like with https://doc.rust-lang.org/stable/std/collections/struct.HashMap.html#method.insert

At the moment we have to do a get before the insert.