Rollup merge of #40559 - nagisa:manually-drop, r=alexcrichton

Implement Manually Drop

As the RFC has been from approx a week in FCP without any major comments, I’m taking the opportunity to submit the PR early.

diff --cc src/doc/unstable-book/src/SUMMARY.md
Simple merge

diff --cc src/libcollections/lib.rs
Simple merge

diff --cc src/librustc_trans/intrinsic.rs
Simple merge