Move native signatures out of Module (#2362)

After compilation there's actually no need to hold onto the native
signature for a wasm function type, so this commit moves out the
`ir::Signature` value from a `Module` into a separate field that's
deallocated when compilation is finished. This simplifies the
`SignatureRegistry` because it only needs to track wasm functino types
and it also means less work is done for `Func::wrap`.

crates/jit/src/compiler.rs

@@ -158,8 +158,7 @@ impl Compiler {
             vec![]
         };
 
-        let (obj, unwind_info) =
-            build_object(&*self.isa, &translation.module, &funcs, dwarf_sections)?;
+        let (obj, unwind_info) = build_object(&*self.isa, &translation, &funcs, dwarf_sections)?;
 
         Ok(Compilation {
             obj,