Compilation is broken on nightly

[lovasoa]

Compilation using the latest nightly (1.37.0-nightly (17e62f77f 2019-07-01)) emits warnings, and this prevents compilation because of #![deny(warnings)].

[lovasoa]

The problem with #![deny(warnings)] is well explained here, together with working alternatives: https://github.com/rust-unofficial/patterns/blob/master/anti_patterns/deny-warnings.md

[seanmonstar]

That's ok, we can just fix the code triggering a warning (I disagree that it's an anti-pattern).

[lovasoa]

The warnings are compatibility warnings. For instance:

error: trait objects without an explicit `dyn` are deprecated

So, "fixing" the code would mean switching to the dyn syntax, and that would break compatibility with rust<1.27. Allowing compatibility warnings (not all warnings) would keep the code compatible with both rust 1.20 and nightly.

[seanmonstar]

I think the new warning lint didn't exist in 1.20 either, so probably just need an #[allow(warnings)] in the few places this happens. It's probably the extensions map, right?

[carllerche]

Fixed by #329.