Reduce the number of clippy warnings

In an attempt to remove (or suppress) all issues pointed out by clippy, this is a first set of changes to reduce the number of warnings.

These are only the simple and hopefully indisputable changes and I would like to get them reviewed before looking at the remaining warnings. The changes are made in small commits, one for each type of warning, so if you dislike any changes, it should be simple to drop them from this pull request.

Eventually it would be nice to enforce a no-warnings-from-clippy policy for merges to master. Same for code-is-formatted-according-to-rustfmt.