Style fixes

[samueltardieu]

Apply various style fixes found by Clippy:

• Do not deconstruct an error to rebuild it right after
• Remove useless as usize
• Use char rather than &str as starts_with() argument
• Remove useless references

[samueltardieu]

I have separated the style fixes into various commits so that some could be removed if the maintainers prefer to let the code this way. This can be the case for the "Remove useless as usize" for example, as even though I don't find !0 as usize more readable than !0 (or usize::MAX as I would have used myself), some others may differ.

[samueltardieu]

Rebased on current master branch.