Change DocFragments from enum variant fields to structs with a nested enum

This makes the code a lot easier to work with. It also makes it easier
to add new fields without updating each variant and `match`
individually.

- Name the `Kind` variant after `DocFragmentKind` from `collapse_docs`
- Remove unneeded impls

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -232,7 +232,12 @@ impl fold::DocFolder for CoverageCalculator {
                 let mut tests = Tests { found_tests: 0 };
 
                 find_testable_code(
-                    &i.attrs.doc_strings.iter().map(|d| d.as_str()).collect::<Vec<_>>().join("\n"),
+                    &i.attrs
+                        .doc_strings
+                        .iter()
+                        .map(|d| d.doc.as_str())
+                        .collect::<Vec<_>>()
+                        .join("\n"),
                     &mut tests,
                     ErrorCodes::No,
                     false,