fix needless lifetimes clippy findings

[cpu]

Fixes errors from clippy nightly of the form:

error: the following explicit lifetimes could be elided: 'a
  --> src/panic.rs:43:6
   |
43 | impl<'a> Defaultable for rustls_slice_bytes<'a> {}
   |      ^^                                     ^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes
   = note: `-D clippy::needless-lifetimes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::needless_lifetimes)]`
help: elide the lifetimes