Added common traits, `clippy`, `rustfmt`, deps,

[virtualritz]

• Added Clone & Debug traits to ensure drop-in compatibility with std containers.
• Made clippy (almost) happy:

      Checking gxhash v2.2.3 (/home/moritz/code/crates/gxhash)
  warning: module has the same name as its containing module
    --> src/gxhash/platform/mod.rs:11:1 
     |
  11 | pub mod platform;
     | ^^^^^^^^^^^^^^^^^
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#module_inception
     = note: `#[warn(clippy::module_inception)]` on by default

• Ran rustfmt.
• Bumped (dev-) dependencies.