As Matklad explains, crates.io is the community repository. Everyone is welcome to submit new crates/libraries there, if they believe they could be useful to other people. Small, targeted crates are not a problem. (e.g. md-5, whose only purpose is to provide MD5 hashing)
If you have a small, specific problem with an existing library, most authors react very positive to code contributions. Crates.io in general will have the links to the source code repository if each individual crate/library.
Practically all libraries on crates.io are open source, so you can open issues/bugs and pull requests for code contribution at each crates individual repository.
If you’re talking about the standard library, changes there go through the Rust RFC process. This usually starts with some informal discussion here on these forums to collect opinions. Then somebody (anyone goes, even you and me) writes an RFC (“request for comment”) as an issue in the RFC repository, and discussion continues there.
This more formal process reflects the increased standard for the standard library, which is supposed to maintain API stability for decades