Work around async_trait behavior

The `#[async_trait]` attribute/crate does a transformation to all async
methods, which as far as i can tell removes the `&self` lifetime, so our
logic to offset by 1 if `has_self == true` for both compared types has
an off-by-one-error here. This didn't error out earlier, since
`get_region_from_params` uses `Vec::get`, so "this index is out of
bounds" is just as `None` as "this generic param is not of kind
lifetime".

Also this is more a workaround than a fix. I'm not sure if we can do
something cleverer than "check if the last lifetim's name is
`'async_trait`".

src/traverse.rs

@@ -736,8 +736,17 @@ fn diff_generics(
     let old_count = old_gen.own_counts();
     let new_count = new_gen.own_counts();
 
+    // NEEDSWORK: proper detection of what's going on here..
+    let was_async_trait_transformed = old_count.lifetimes > 0
+        && old_gen.params[old_count.lifetimes - 1].name.as_str() == "'async_trait";
+
     let self_add = if old_gen.has_self && new_gen.has_self {
-        1
+        if was_async_trait_transformed {
+            // async_trait transformation undoes the self lifetime?
+            0
+        } else {
+            1
+        }
     } else if !old_gen.has_self && !new_gen.has_self {
         0
     } else {