* [core] Add Entry API for storage2::HashMap

* [core] Replace unwrap's with expect's

* [core] Shorten code

* [core] Improve tests

* [core] Improve tests

* [core] Add benches for storage2::HashMap Entry API

* [core] Update comments

* [core] Add + update benches

* [core] Improve benches code, add more benches

* [core] Improve expect messages

* [core] Simplify test structure

* [core] Implement review comments

* [core] Shorten bench code

* [core] Fix bench iterations

* [core] Minimze black_box'es

* [core] Remove code dups

* [core] Shorten code