Auto merge of #13793 - heycam:cleanup-2, r=Manishearth

Make use statements in gecko.mako.rs neater.

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Similarly to #13792 I find the grouped `use` statements in gecko.mako.rs hard to deal with.  Splitting these out to one per line should help avoid rebasing problems (which I sometimes get), is easier to read, and easier to keep sorted.  r? @emilio

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