Provide Natvis using `debugger_visualizer` attribute

[obsgolem]

Rust has a relatively new unstable attribute for embedding debugger visualization info. Something like that would be useful to provide with slotmap. See https://rust-lang.github.io/rfcs/3191-debugger-visualizer.html

[orlp]

I'll let the feature marinate for a bit, but once the rough edges are off and it's being kept at least as unstable I'll add it to slotmap.