Add a lint for panic! being used

[sgrif]

Given that there's a lint for Result::unwrap and Option::unwrap being used, I was surprised that this wasn't present.

[y21]

This lint exists today (along with lints for all the other kinds of panic-like macros).

#[warn(clippy::panic)]
fn f() {
    panic!();
}

warning: `panic` should not be present in production code
 --> src/main.rs:3:5
  |
3 |     panic!();
  |     ^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#panic