Adds a lints section to the top of lib.rs with the following:

    #![warn(
        clippy::unwrap_used,
        missing_docs,
        rust_2018_idioms,
        unused_lifetimes,
        unused_qualifications
    )]

`warn` is used instead of `deny` to prevent the lints from firing during
local development, however we already configure `-D warnings` in CI so
if any lint fails on checked-in code, it will cause a CI failure.

This commit also fixes or explicitly allows any current violations of
these lints. The main ones were:

- `clippy::unwrap_used`: replaces usages of `unwrap` with `expect`
- `rust_2018_idioms`: no implicit lifetimes, which were present on
  usages of `core::fmt::Formatter`