Remove unused `desc` attribute from crates

The `desc` crate attribute has been removed from Rust.
You can use the `description` field in Cargo.toml to describe your package.

This commit removes the desc attribute from all crates.
A `description` field in Cargo.toml is added to describe the package.
Merge pull request #63 from crumblingstatue/remove_desc

Remove unused `desc` attribute from crates