Resolve the majority of pedantic clippy warnings

[andypymont]

Resolve most warnings shown by cargo clippy -- -W clippy::pedantic:

• clippy::single_match_else
• clippy::if_not_else
• clippy::must_use_candidate
• clippy::module_name_repetitions

This leaves only the Panics/Errors documentation warnings as discussed in #28

[fspoettel]

Hey, thanks for getting the ball rolling here. I merged this as https://github.com/fspoettel/advent-of-code-rust/commit/d7af3dca9e52f9868b346d4058fe68248fb8950b after fixing some merge conflicts and credited you as co-author.