devel/rust-analyzer: Update to 2021-03-29

Changes: https://rust-analyzer.github.io/thisweek/2021/03/15/changelog-68.html
Changes: https://rust-analyzer.github.io/thisweek/2021/03/22/changelog-69.html
Changes: https://rust-analyzer.github.io/thisweek/2021/03/29/changelog-70.html