Add a "Discard" type

[mitsuhiko]

I'm currently encountering the issue that I want to selectively discard data of maps. Effectively I have a deeply nested JSON structure and at one point all I care about is the keys of a map but not the values. So ideally one could use something like a serde::Discard as a stand-in that's implemented similarly to serde_json::Value but completely discards all values while still parsing correctly.

[oli-obk]

this could also just be a helper function for deserialize_with #553

[dtolnay]

This exists as serde::de::impls::IgnoredAny.

[dtolnay]

Closing because I see you used IgnoredAny in getsentry/rust-sourcemap@a665671.

[mitsuhiko]

Sorry yeah, forgot to reply to this. Completely missed it in the docs. Thanks!