Support untagged enum representation

[ghost]

In addition to externally, internally, and adjacently tagged enum representations, serde also supports untagged representation.

It would be nice to have support for untagged representation in typetag too.

[toxeus]

IIUC serde's untagged enum representation basically bruteforces through all possible enum variants until one of them succeeds. To make this work with typetag a list of all types that implement a given trait would need to be collected. I don't know how hard that is. Perhaps @dtolnay has a better understanding on how involved implementing this would be?

[dtolnay]

I would prefer not to support this in this crate. But it would be reasonable for someone else to make a different crate implementing untagged enums.