wiggle: add support for async traits; ABI func is now generic of ctx

* ctx parameter no longer accepted by wiggle::from_witx macro.
* optional async_ parameter specifies which functions are async.
* re-export async_trait::async_trait, so users don't have to take a dep.

crates/wiggle/Cargo.toml

@@ -16,6 +16,7 @@ witx = { path = "../wasi-common/WASI/tools/witx", version = "0.9", optional = tr
 wiggle-macro = { path = "macro", version = "0.23.0" }
 tracing = "0.1.15"
 bitflags = "1.2"
+async-trait = "0.1.42"
 
 [badges]
 maintenance = { status = "actively-developed" }