style: Clean up most comment delimeters

Clean up the majority of Rust's comments so they match the preferred delimeter style.
Discussion @ issue #15458.

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This was done semi-manually (using [ag](https://github.com/ggreer/the_silver_searcher) and vim + macros), so I've had the chance to look at every single comment before judging if I should change it or not.

I left a lot of block comments when they were inside a line, and a couple block doc comments that were extremely large ([`borrowck::doc`](https://github.com/rust-lang/rust/blob/master/src/librustc/middle/borrowck/doc.rs) for example). I also left any block comments that were used in parsing tests, naturally.