style: Remove needless lifetimes

[caspermeijn]

This solves clippy warnings like this:

error: the following explicit lifetimes could be elided: 'a
  --> src/common.rs:30:6
   |
30 | impl<'a> Text<'a> for String {
   |      ^^       ^^
   |
   = 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
   |
30 - impl<'a> Text<'a> for String {
30 + impl Text<'_> for String {