attributes: remove closure type annotation in #[instrument(err)]

Currently, using `#[instrument(err)]` on a function returning a `Result`
with an `impl Trait` in it results in a compiler error. This is because
we generate a type annotation on the closure in the function body that
contains the user function's actual body, and `impl Trait` isn't allowed
on types in local bindings, only on function parameters and return
types.

This branch fixes the issue by simply removing the return type
annotation from the closure. I've also added tests that break on master
for functions returning `impl Trait`, both with and without the `err`
argument.

Fixes #1227

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-attributes/src/lib.rs

@@ -469,7 +469,7 @@ fn gen_body(
         quote_spanned!(block.span()=>
             let __tracing_attr_span = #span;
             let __tracing_attr_guard = __tracing_attr_span.enter();
-            match move || #return_type #block () {
+            match (move || #block)() {
                 #[allow(clippy::unit_arg)]
                 Ok(x) => Ok(x),
                 Err(e) => {