Include doctests for code coverage

[pitdicker]

This requires nightly, see taiki-e/cargo-llvm-cov#2.

[codecov]

Codecov Report

Merging #1248 (2d575b8) into 0.4.x (ecfad9e) will increase coverage by 5.59%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            0.4.x    #1248      +/-   ##
==========================================
+ Coverage   85.82%   91.41%   +5.59%     
==========================================
  Files          37       37              
  Lines       13517    16452    +2935     
==========================================
+ Hits        11601    15040    +3439     
+ Misses       1916     1412     -504     

see 19 files with indirect coverage changes

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[pitdicker]

Code coverage increased from 85.82% to 91.41% 🚀.

[djc]

Nice, that's a pretty cheap win!

Do we know why it needs nightly? Is there some tracking issue I could follow?

[pitdicker]

I am not sure what would be the right issue to track.
https://doc.rust-lang.org/rustc/instrument-coverage.html#including-doc-tests mentions code coverage relies on the -Z unstable-options --persist-doctests. So either that must be stabilized (tracking issue rust-lang/rust#56925), or code coverage must switch to something different.
taiki-e/cargo-llvm-cov#2 seems easier to follow for updates.