Generate macro expansion in rustdoc

[jonasbb]

A recent rust nightly added the ability to show macro expansions in the
rustdoc output. Since this crate uses macros for many trait
implementations, this is useful for users trying to understand the macro
output.
rust-lang/rust#137229

Closes #879

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.60%. Comparing base (f414463) to head (b852142).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #883   +/-   ##
=======================================
  Coverage   68.60%   68.60%           
=======================================
  Files          36       36           
  Lines        9168     9168           
=======================================
  Hits         6290     6290           
  Misses       2878     2878           

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