Figure out consistent code style

I've seen a few of files now in FluxML repos where the diffs ping-pong back and forth between different styles. Most of the discrepancy seems to be around function and type signature whitespace. Trailing whitespace too, my editor config is set to trim them and a surprising number of core files seem to have them.

ChainRulesCore recently added JuliaFormatter to their CI and it seems to be working well, but even just a couple of sentences in CONTRIBUTING.md would go a long way.