add #[derive(Debug)] to Matches struct; run cargo fmt; fix with clippy

[Zaechus]

I added the Debug implementation to the Matches struct and associated data types. I also took the liberty to format the code with rustfmt and fix some small clippy lints. The lint changes shouldn't affect the functionality at all.

Fixes #64

[KodrAus]

Thanks @maxgy!

I gave this a quick scan through, and couldn't spot any cases where clippy might've lead us to make any breaking changes.

[Zaechus]

The clippy lints were only for changing things like
!a.is_none() to a.is_some() or
a.len() > 0 to !a.is_empty() or
Err(_) to _,
so the library should behave the same way.