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/faerie/src/lib.rs

@@ -27,9 +27,8 @@
 
 mod backend;
 mod container;
-pub mod traps;
 
-pub use crate::backend::{FaerieBackend, FaerieBuilder, FaerieProduct, FaerieTrapCollection};
+pub use crate::backend::{FaerieBackend, FaerieBuilder, FaerieProduct};
 pub use crate::container::Format;
 
 /// Version number of this crate.