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/object/src/backend.rs

@@ -17,6 +17,7 @@ use object::write::{
 use object::{RelocationEncoding, RelocationKind, SymbolFlags, SymbolKind, SymbolScope};
 use std::collections::HashMap;
 use std::mem;
+use std::ops::IndexMut;
 use target_lexicon::{BinaryFormat, PointerWidth};
 
 #[derive(Debug)]
@@ -188,7 +189,7 @@ impl Backend for ObjectBackend {
         ctx: &cranelift_codegen::Context,
         _namespace: &ModuleNamespace<Self>,
         code_size: u32,
-    ) -> ModuleResult<ObjectCompiledFunction> {
+    ) -> ModuleResult<(ObjectCompiledFunction, Option<&Vec<TrapSite>>)> {
         let mut code: Vec<u8> = vec![0; code_size as usize];
         let mut reloc_sink = ObjectRelocSink::new(self.object.format());
         let mut trap_sink = ObjectTrapSink::default();
@@ -229,8 +230,9 @@ impl Backend for ObjectBackend {
                 relocs: reloc_sink.relocs,
             });
         }
-        self.traps[func_id] = trap_sink.sites;
-        Ok(ObjectCompiledFunction)
+        let trapref = self.traps.index_mut(func_id);
+        *trapref = trap_sink.sites;
+        Ok((ObjectCompiledFunction, Some(trapref)))
     }
 
     fn define_function_bytes(
@@ -240,14 +242,15 @@ impl Backend for ObjectBackend {
         bytes: &[u8],
         _namespace: &ModuleNamespace<Self>,
         traps: Vec<TrapSite>,
-    ) -> ModuleResult<ObjectCompiledFunction> {
+    ) -> ModuleResult<(ObjectCompiledFunction, Option<&Vec<TrapSite>>)> {
         let symbol = self.functions[func_id].unwrap();
         let section = self.object.section_id(StandardSection::Text);
         let _offset = self
             .object
             .add_symbol_data(symbol, section, bytes, self.function_alignment);
-        self.traps[func_id] = traps;
-        Ok(ObjectCompiledFunction)
+        let trapref = self.traps.index_mut(func_id);
+        *trapref = traps;
+        Ok((ObjectCompiledFunction, Some(trapref)))
     }
 
     fn define_data(