change Module::define_function to take TrapSink instances

Experience with the `define_function` API has shown that returning
borrowed slices of `TrapSite` is not ideal: the returned slice
represents a borrow on the entire `Module`, which makes calling back
into methods taking `&mut self` a bit tricky.

To eliminate the problem, let's require the callers of `define_function`
to provide `TrapSink` instances.  This style of API enables them to
control when and how traps are collected, and makes the `object` and
`faerie` backends simpler/more efficient by not having to worry about
trap collection.

cranelift/simplejit/examples/simplejit-minimal.rs

@@ -1,4 +1,5 @@
 use cranelift::prelude::*;
+use cranelift_codegen::binemit::NullTrapSink;
 use cranelift_module::{default_libcall_names, Linkage, Module};
 use cranelift_simplejit::{SimpleJITBackend, SimpleJITBuilder};
 use std::mem;
@@ -38,7 +39,10 @@ fn main() {
         bcx.seal_all_blocks();
         bcx.finalize();
     }
-    module.define_function(func_a, &mut ctx).unwrap();
+    let mut trap_sink = NullTrapSink {};
+    module
+        .define_function(func_a, &mut ctx, &mut trap_sink)
+        .unwrap();
     module.clear_context(&mut ctx);
 
     ctx.func.signature = sig_b;
@@ -60,7 +64,9 @@ fn main() {
         bcx.seal_all_blocks();
         bcx.finalize();
     }
-    module.define_function(func_b, &mut ctx).unwrap();
+    module
+        .define_function(func_b, &mut ctx, &mut trap_sink)
+        .unwrap();
     module.clear_context(&mut ctx);
 
     // Perform linking.