Update displaydoc to 0.2 or replace it with thiserror

[mbelop]

Displaydoc 0.2 now fails hard when multi-line doc comments are used because it does not and has never supported those. It would be nice to update the dependency to 0.2 or even better replace it with thiserror.

[mbelop]

Or replace it with a hand-rolled impl: #4

[mbelop]

Bump. @mbr, can you please take a look at this?