Fix clippy warnings

[barafael]

Building upon the changes #134 and #135, a bunch of new clippy lints surfaced.

I fixed all the default clippy lints and chose a good selection (according to me) of the lints in clippy::pedantic and clippy::nursery.

Many changes are very simple, such as using struct initializer shorthand or let-else instead of let-if-let.

Let me know what you think.