wiggle::async_trait is defined as async_trait::async_trait(?Send)

async methods used by wiggle currently need to Not have the Send
constraint, so rather than make all use sites pass the argument
to the re-exported async_trait macro, define a new macro that
applies the argument.

crates/wiggle/macro/Cargo.toml

@@ -25,6 +25,7 @@ wiggle-generate = { path = "../generate", version = "0.25.0" }
 witx = { version = "0.9.0", path = "../../wasi-common/WASI/tools/witx" }
 quote = "1.0"
 syn = { version = "1.0", features = ["full"] }
+proc-macro2 = "1.0"
 
 [dev-dependencies]
 wiggle = { path = ".." }