Implemented slab storage that allocates handles (mozilla#1730)

This will be used for passing handles across the FFI.

Use proptest to test the Rust implementation. Created a new fixture to
test foreign bindings code for this.

The docs list list of advantages to this, the main disadvantage is
performance. We need to allocate a vec entry for the pointer and
insert/remove/get need to take a lock.

Compared `Arc<ConcreteType>` this is clearly worse. However, compared to
`Arc<dyn Trait>` it's not so clear when we currently box the arc
before passing it across the FFI.  Compared to the foreign HandleMaps, I
think the new system will be faster.

It seems reasonable to trade some performance for the gains in
simplicity and safety.  The one thing that really bugs me is that
`get()` needs to take a read lock.  I'm pretty sure there's a couple
ways to fix this, but I'm not sure if it's worth the complexity.

Cargo.toml

@@ -22,6 +22,7 @@ members = [
   "examples/traits",
 
   "fixtures/benchmarks",
+  "fixtures/bindings-internal",
   "fixtures/coverall",
   "fixtures/callbacks",