Migrate to in-progress version of FFI backend 0.6.0

Includes:
- Multi-step transaction proposals.
- Changes to support `Synchronizer.proposeShielding` API changes.

Example/ZcashLightClientSample/ZcashLightClientSample.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved

@@ -176,8 +176,7 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/zcash-hackworks/zcash-light-client-ffi",
       "state" : {
-        "revision" : "c90afd6cc092468e71810bc715ddb49be8210b75",
-        "version" : "0.5.1"
+        "revision" : "789d0c068fb32e2ab149cdd785f16e0ac88f3594"
       }
     }
   ],

Package.resolved

@@ -122,8 +122,7 @@
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/zcash-hackworks/zcash-light-client-ffi",
       "state" : {
-        "revision" : "c90afd6cc092468e71810bc715ddb49be8210b75",
-        "version" : "0.5.1"
+        "revision" : "789d0c068fb32e2ab149cdd785f16e0ac88f3594"
       }
     }
   ],

Package.swift

@@ -16,7 +16,8 @@ let package = Package(
     dependencies: [
         .package(url: "https://github.com/grpc/grpc-swift.git", from: "1.19.1"),
         .package(url: "https://github.com/stephencelis/SQLite.swift.git", from: "0.14.1"),
-        .package(url: "https://github.com/zcash-hackworks/zcash-light-client-ffi", exact: "0.5.1")
+        // Compiled from revision `97e09ed3709ae9f26226587bec852a725bc783a4`.
+        .package(url: "https://github.com/zcash-hackworks/zcash-light-client-ffi", revision: "789d0c068fb32e2ab149cdd785f16e0ac88f3594")
     ],
     targets: [
         .target(

Sources/ZcashLightClientKit/Model/Proposal.swift

@@ -13,7 +13,9 @@ public struct Proposal: Equatable {
 
     /// Returns the total fee to be paid across all proposed transactions, in zatoshis.
     public func totalFeeRequired() -> Zatoshi {
-        Zatoshi(Int64(inner.balance.feeRequired))
+        inner.steps.reduce(Zatoshi.zero) { acc, step in
+            acc + Zatoshi(Int64(step.balance.feeRequired))
+        }
     }
 }