Remove the excessive `#[allow(clippy::missing_inline_in_public_items)]`

[jonas-schievink]

These were added in #175 because the lint fires on #[derive(Debug)]-generated impls. Once rust-lang/rust-clippy#4861 is fixed on stable, the #[allow]s should be removed from all structs.

[jonas-schievink]

Upstream bug has been fixed, the first Rust version with the fix will likely be 1.41