rustdoc: reference function signature types from the p array

This reduces the size of the function signature index, because
it's common to have many functions that operate on the same types.

    $ wc -c search-index-old.js search-index-new.js
    5224374 search-index-old.js
    3932314 search-index-new.js

By my math, this reduces the uncompressed size of the search index by 32%.
On compressed signatures, the wins are less drastic, a mere 8%:

    $ wc -c search-index-old.js.gz search-index-new.js.gz
    404532 search-index-old.js.gz
    371635 search-index-new.js.gz

src/librustdoc/json/conversions.rs

@@ -753,7 +753,6 @@ impl FromWithTcx<ItemType> for ItemKind {
             TraitAlias => ItemKind::TraitAlias,
             ProcAttribute => ItemKind::ProcAttribute,
             ProcDerive => ItemKind::ProcDerive,
-            Generic => unreachable!(),
         }
     }
 }