`no_std` support

[Oakchris1955]

Short Description:

Support for building and running duplicate in no_std environments

Motivation:

The library itself is great, it is however a pity it doesn't support no_std targets. While the majority of Rust code that is written depends on the standard library one way or another, I don't see a reason why this one, with its simple design and purpose, shouldn't have support for no_std environments.

Design

Replace all std use statements with core and alloc ones and any use of HashMaps and HashSets with ones from the hashbrown crate.

Misc:

Since either case we need to depend on Rust's alloc library, perhaps that should be mentioned?

[Emoun]

Thank you for the interest in this crate.

What is the purpose of this change? duplicate is a compile-time only crate, so the fact that it uses std should not limit its use for code that is used in a no_std environments. If I'm misunderstanding, please give me an example use case where this is an issue (especially if its an issue for you).

[Oakchris1955]

That... seems to be right? I had been using another crate prior to duplicate and that one for whatever reason refused to compile in a no_std environment, so when I found duplicate I didn't even check if that was the case here.