Try to clean up some lints and enable more

[psychon]

I came across the unused_extern_crates lint, which is allow-by-default. It is however part of the rust_2018_idioms lint group, which we are already denying.

Anyway, this discovery led me down a linting rabbit hole.

There is the unused_import_braces lint which arguable lints against something that rustfmt already deals with. Both the lint and rustfmt deal with the braces in "use test::{A};". Thus, this lint can just be dropped.

The "unused" lint group includes "unused_must_use" and some other things that sound helpful, so let's just switch to the more generic lint. This found some unused_macro_rules in x11rb/src/tracing.rs, which need to be allowed, so this lint can just be denied and not forbidden. But some of the lints that we forbid are part of the "unused" group and #[deny]ing something that was previously #[forbid]en is an error. Thus, I am switching the order of these and move all the #[deny]s up.

Next, I thought that "unused_qualifications" should be part of "unused", but it actually is not. The following code warns about the allows for unused_extern_crates and unused_must_use, but not unused_qualifications, so this is not part of the unused group. Same for unused_results.

#![forbid(unused)]
#![allow(unused_extern_crates)]
#![allow(unused_must_use)]
#![allow(unused_qualifications)]
#![allow(unused_results)]

---

I now feel confused about lints. Is this PR still a good idea? Does it improve anything?

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 13.06%. Comparing base (08ce9fa) to head (6551db1).

Files	Patch %	Lines
x11rb/src/properties.rs	0.00%	2 Missing ⚠️
x11rb-protocol/src/resource_manager/mod.rs	0.00%	1 Missing ⚠️
x11rb/src/rust_connection/packet_reader.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #923      +/-   ##
==========================================
- Coverage   13.06%   13.06%   -0.01%     
==========================================
  Files         191      191              
  Lines      136668   136678      +10     
==========================================
  Hits        17860    17860              
- Misses     118808   118818      +10     

Flag	Coverage Δ
tests	13.06% <33.33%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.