offer function-level control over tracing (#5194)

* wiggle: fix compilation with async functions when tracing is off

Fixes #5202

* switch tracing config from a boolean to a struct

This will enable more complex tracing rules in the future

* rename AsyncConfField to FunctionField

It is going to be reused for cases other than just async functions

* add support for disabling tracing per-function

This adds a `disable_for` syntax after the `tracing` boolean.  For
example:

```
wiggle::from_witx!(
    tracing: true disable_for {
        module1::foo,
        module2::{bar, baz},
    }
)
```

crates/wiggle/generate/src/codegen_settings.rs

@@ -1,4 +1,4 @@
-use crate::config::{AsyncConf, ErrorConf};
+use crate::config::{AsyncConf, ErrorConf, TracingConf};
 use anyhow::{anyhow, Error};
 use proc_macro2::TokenStream;
 use quote::quote;
@@ -15,7 +15,7 @@ pub struct CodegenSettings {
     // Disabling this feature makes it possible to remove all of the tracing
     // code emitted in the Wiggle-generated code; this can be helpful while
     // inspecting the code (e.g., with `cargo expand`).
-    pub tracing: bool,
+    pub tracing: TracingConf,
 }
 impl CodegenSettings {
     pub fn new(
@@ -23,14 +23,14 @@ impl CodegenSettings {
         async_: &AsyncConf,
         doc: &Document,
         wasmtime: bool,
-        tracing: bool,
+        tracing: &TracingConf,
     ) -> Result<Self, Error> {
         let errors = ErrorTransform::new(error_conf, doc)?;
         Ok(Self {
             errors,
             async_: async_.clone(),
             wasmtime,
-            tracing,
+            tracing: tracing.clone(),
         })
     }
     pub fn get_async(&self, module: &Module, func: &InterfaceFunc) -> Asyncness {