cranelift-module: expose trap information when defining functions

The current interface of `cranelift-module` requires consumers who want
to be informed about traps to discover that information through
`Module::Product`, which is backend-specific.  Since it's advantageous
to manipulate this information in a backend-agnostic way, this patch
changes `Module::define_function{,_bytes}` to return information about
the traps contained in the function being defined.

cranelift/faerie/src/traps.rs

@@ -59,7 +59,8 @@ impl FaerieTrapManifest {
     }
 
     /// Put a `FaerieTrapSink` into manifest
-    pub fn add_sink(&mut self, sink: FaerieTrapSink) {
+    pub fn add_sink(&mut self, sink: FaerieTrapSink) -> &Vec<TrapSite> {
         self.sinks.push(sink);
+        &self.sinks.last().unwrap().sites
     }
 }