Expand lints

[tarcieri]

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
• unused_lifetimes: corrects a few instances

[rozbb]

These are all good lints imo. Ty