Make clippy an optional dep

[Manishearth]

This way people can use this crate on stable. Compile with cargo build --features dev to use clippy

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[Manishearth]

Needs autumnai/phloem#15

[MichaelHirn]

As @Manishearth stated, we should make cuticulla complay with the usual clippy handling we have at the other crates. But phloem needs to be replaced first with collenchyma.

I cc @hobofan, maybe he has a local branch for this.

Progress is tracked at #19 which blocks the merge of this PR.

[homu]

☔ The latest upstream changes (presumably #21) made this pull request unmergeable. Please resolve the merge conflicts.