Introduce Builder and Product types to the Module workflow.

This eliminates API confusion and surface area with respect to what state
the `Backend` needs to be in at different points.

Now, API users will construct a `Builder`, and pass it into the `Module`
which uses it to constrct a `Backend`. The `Backend` instance only lives
inside the `Module`. And when finished, the `Module` can return a
`Product` back to the user providing any outputs it has.

lib/faerie/src/lib.rs

@@ -29,5 +29,5 @@ mod backend;
 mod container;
 mod target;
 
-pub use backend::FaerieBackend;
+pub use backend::{FaerieBuilder, FaerieBackend};
 pub use container::Format;