rustdoc: rename /implementors to /impl.trait

This is shorter, avoids potential conflicts with a crate
named `implementors`[^1], and will be less confusing when JS
include files are added for type aliases.

[^1]: AFAIK, this couldn't actually cause any problems right now,
but it's simpler just to make it impossible than relying on never
having a file named `trait.Foo.js` in the crate data area.

src/librustdoc/html/render/write_shared.rs

@@ -362,7 +362,7 @@ if (typeof exports !== 'undefined') {exports.searchIndex = searchIndex};
     }
 
     // Update the list of all implementors for traits
-    let dst = cx.dst.join("implementors");
+    let dst = cx.dst.join("trait.impl");
     let cache = cx.cache();
     for (&did, imps) in &cache.implementors {
         // Private modules can leak through to this phase of rustdoc, which