Chore: remove excess dependencies

[BastiDood]

This PR aims to reduce the number of dependencies for both development and production environments.

Production Dependencies

Before, on my machine, building the library requires pulling in 49 crates. This is not so bad, but when I looked into the project manifest, I realized that a huge bulk of the bloat comes from the futures crate.

Hence, the first two commits of this PR addresses this issue by replacing the futures crate for the much more lightweight futures-core crate in production environments. The futures-core crate provides the most minimal interface for the Stream trait.

Overall, this reduces the number of built crates from 49 to 22. That's a significant improvement! This was mostly due to the fact that the futures crate packages an executor by default (among other unused imports), whereas the futures-core crate only exposes core traits. Since bmrng only used the Stream interface, there is no need for the executor (and other imports).

Development Dependencies

Meanwhile, following the removal of the futures crate in all environments, the futures-util crate (which depends on futures_core) is used to regain access to the StreamExt utilities for tests.

Finally, the development dependency on tokio-test has also been removed. The tests only used tokio-test for the assert_ok and assert_err macros, which may easily be emulated by the built-in assert macro with the Result::is_{ok|err} methods.

While I was there in the tests, I also refactored multiple instances of redundant Boolean assertions. That is:

#[test]
fn it_works() {
    // ...
    // This is too verbose.
    assert_eq!(tx.is_closed(), false);

    // This does the same thing,
    // but is definitely more concise
    // and less redundant.
    assert!(!tx.is_closed());
}

If there are any issues with this PR, please do let me know. I will gladly resolve them! Thanks!

[oguzbilgener]

Hi there, thank you for the PR! This is certainly a significant improvement in build times!
I have no problem with removing tokio-test either.

[oguzbilgener]

I guess this was unused 👍