dbg! prints can tear in multi-threading code

[orlp]

This is technically not a bug, but simply a rather frustrating behavior. If you write dbg!(a, b), then the implementation doesn't take a lock on stderr for the entire debug print, but for a and b separately. This means that they can tear with other (debug) prints on stderr. E.g. if you have

 // Thread A.
let (a, b) = (1, 2);
dbg!(a, b);

 // Thread B.
let (a, b) = (3, 4);
dbg!(a, b);

You can see the output (or various other interleavings, it gets worse with more statements and more threads)

[src/some/path.rs] a = 3
[src/some/path.rs] a = 1
[src/some/path.rs] b = 4
[src/some/path.rs] b = 2

A workaround is to use dbg!((a, b)) but I'd rather we just fix the implementation to take a lock once for the entire dbg! call to make the entire debug print atomic.