Test nullifiers in constant time

Checking for spent notes in a block is still not completely constant
time, due to filtering out negative results of the constant-time
comparison.

Part of #84.

zcash_client_backend/Cargo.toml

@@ -17,6 +17,7 @@ ff = { version = "0.5.0", path = "../ff" }
 hex = "0.3"
 pairing = { version = "0.15.0", path = "../pairing" }
 protobuf = "2"
+subtle = "2"
 zcash_primitives = { version = "0.1.0", path = "../zcash_primitives" }
 
 [build-dependencies]