attributes: update #[instrument] to support async-trait 0.1.43+ (#…

…1228)

This backports #1228 from `master` to `v0.1.x`.

It works with both the old and new version of async-trait (except for
one doc test that failed previously, and that works with the new
version). One nice thing is that the code is simpler (e.g.g no self
renaming to _self, which will enable some simplifications in the
future).

A minor nitpick is that I disliked the deeply nested pattern matching in
get_async_trait_kind (previously: get_async_trait_function), so I
"flattened" that a bit.

Fixes  #1219.

tracing-attributes/Cargo.toml

@@ -39,15 +39,14 @@ async-await = []
 
 [dependencies]
 proc-macro2 = "1"
-syn = { version = "1", default-features = false, features = ["full", "parsing", "printing", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
+syn = { version = "1", default-features = false, features = ["full", "parsing", "printing", "visit", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
 quote = "1"
 
-
 [dev-dependencies]
 tracing = { path = "../tracing", version = "0.1" }
 tokio-test = { version = "0.2.0" }
 tracing-core = { path = "../tracing-core", version = "0.1"}
-async-trait = "0.1"
+async-trait = "0.1.44"
 
 [badges]
 maintenance = { status = "experimental" }