fix: make the empty array CID const and export it (filecoin-project#668)

Otherwise, we'll perform a hash in WASM when we first dereference (max
once per call) which is just wasteful.

This also fixes the integration tests to use the "correct" empty object.
Previously, they were using `()` which mapped to null, not `[]`.

Author: Stebalien

runtime/src/runtime/mod.rs

@@ -42,6 +42,9 @@ pub(crate) mod hash_algorithm;
 pub(crate) mod empty;
 pub use empty::EMPTY_ARR_CID;
 
+pub(crate) mod empty;
+pub use empty::EMPTY_ARR_CID;
+
 /// Runtime is the VM's internal runtime object.
 /// this is everything that is accessible to actors, beyond parameters.
 pub trait Runtime<BS: Blockstore>: Primitives + Verifier + RuntimePolicy {