Backport a set of fixes to 1.4.x

[helaan]

As requested in #2148, backport some commits to fix the documentation error. As requested in that commit, the SQLite bump is also included, as are a set of commits to make Diesel compile without warning on 1.38.0

[weiznich]

I see now that libsqlite-sys 0.16 is released. Maybe we should bump that crate again to 0.16 before the release. Can you do add that to this PR?

[helaan]

I've bumped libsqlite3-sys in #2183, will backport that when that one is merged.

[helaan]

Are there any other commits suitable for a backport?

[weiznich]

I've just realized that this PR is not #2175 so I've just drop the list from there here 🙈

#2019
#2056
#2108
#2102
#2164

I think #2019, #2056 and #2164 are missing. It would be great to also get them.

[helaan]

I've incorporated #2019 and #2056, however #2164 requires additional work, which I'm not sure are worth it at this point, as Diesel doesn't expose types from syn and users of syn-1.0 libraries can just pull in another version of syn.

[weiznich]

I've added #2164 to minimize duplicated syn versions (and therefore build time) in complexer projects.