Rollup merge of #54333 - steveklabnik:update-book, r=frewsxcv

Update The Book to latest

Let's check out https://github.com/rust-lang/book/pull/1505 on nightly.
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kennytm 2018-09-20 21:36:31 +08:00
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Subproject commit cff0930664b688f1dd22aefb3d16944eb4cdbfd5
Subproject commit fa91738b66367b6f70b078251868a071f1991ace