chore: Fix a couple broken book links
While I was fixing these, I figured maybe it's good to just use a consistent URL for book links: https://doc.rust-lang.org/book/
Is there a plan for what to do with this old 1.4.0 example that doesn't exist in the current version of the book? Perhaps at least include a disclaimer when recommending that folks check it out?
34e31232df/exercises/threads/README.md (L1)
chore: Clean up some formatting in exercises
I noticed some formatting that isn't consistent with the `rustfmt` style and tried my best to run it on the files in the exercises directory (which does fail for files that can't compile!), which just caught some minor whitespace things here and there.
Note: also can't just apply `rustfmt` blindly because of the blank lines that lead to the hint comments
Fixes the irrefutable let pattern warning in `structs1.rs`
PR https://github.com/rust-lang/rustlings/pull/163 accidentally introduced an error using some versions of the Rust compiler where the compiler would (rightly!) complain about an irrefutable let pattern. I have no idea why this did not occur in all versions of the compiler, but here is a way around it.
Sub READMEs contain the relevant resources from the main
README of the web version. All second edition Rust Book links
were updated to the 2018 edition.
Fixed to typos in main README.md.
