Check if key in UstrMap without creating new ustr

[sharon8811]

We need a way to check if key is in the UstrMap without actually creating ustr,
think of a user input and i want to check if the search term is in the map i will need to keep this user input string in memory forever
If we had a way to check without creating the ustr in memory it would have been perfect

[orzogc]

You could use the raw entry API from HashMap(use hashbrown::HashMap instead in stable Rust).