Improve outer attribute attachment

[wetneb]

This is another take on #244, which makes sure that outer attributes are properly attached to the elements they annotate.
It brings the grammar closer to rustc, removing 7 cases of inconsistencies noted in #229.

25,27d1
< coverage/attr/nested.rs
< coverage/coroutine.rs
< coverage/coverage_attr_closure.rs
29,31d2
< coverage/yield.rs
< debuginfo/coroutine-locals.rs
< debuginfo/coroutine-objects.rs
36d6
< debuginfo/issue-57822.rs

Closes #244. Closes #276.

[wetneb]

If anyone else is interested in this, the improvement is available in the tree-sitter-rust-orchard fork.