Switch to tikz-nfold for double arrows

[varkor]

Currently, https://github.com/jonschz/tikz-nfold is used in quiver only for triple arrows or higher. However, it has better rendering than Rightarrow even for double arrows. It would therefore be preferable always to make use of it. At the moment, however, tikz-nfold isn't present in the TeXLive distribution used by Overleaf, which I imagine many quiver users make use of. I would therefore prefer to wait until Overleaf updates their TeXLive distribution before making it default in quiver.