[#1332] Enable Swiftlint workflow

Closes #1332.

This runs an ubuntu box with a swiftlint process and nothing else.

During implementing this same thing in the ZIP-321 library I found
that the Swift Package Plugin for Swiftlint did work after some
juggling, but it added a lot of build time to the main build job
so it delayed the CI for several minutes while a separate machine
would to the same chore more effectively and also in an isolated
way without mixing lint and build failures

.github/workflows/swiftlint.yml

@@ -0,0 +1,17 @@
+name: SwiftLint
+
+on:
+  pull_request:
+    paths:
+      - '.github/workflows/swiftlint.yml'
+      - '.swiftlint.yml'
+      - '**/*.swift'
+
+jobs:
+  SwiftLint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: GitHub Action for SwiftLint
+        uses: norio-nomura/action-swiftlint@3.2.1
+        

Sources/ZcashLightClientKit/Checkpoint/BundleCheckpointSource.swift

@@ -31,7 +31,7 @@ struct BundleCheckpointSource: CheckpointSource {
     
     func birthday(for height: BlockHeight) -> Checkpoint {
         Checkpoint.birthday(
-            with: height, 
+            with: height,
             checkpointDirectory: BundleCheckpointURLProvider.default.url(self.network)
         ) ?? saplingActivation
     }

Sources/ZcashLightClientKit/Modules/Service/LightWalletService.swift

@@ -137,10 +137,6 @@ protocol LightWalletServiceResponse {
 struct LightWalletServiceFactory {
     let endpoint: LightWalletEndpoint
 
-    init(endpoint: LightWalletEndpoint) {
-        self.endpoint = endpoint
-    }
-
     func make() -> LightWalletService {
         return LightWalletGRPCService(endpoint: endpoint)
     }

Sources/ZcashLightClientKit/Rust/ZcashKeyDerivationBackend.swift

@@ -11,10 +11,6 @@ import libzcashlc
 struct ZcashKeyDerivationBackend: ZcashKeyDerivationBackendWelding {
     let networkType: NetworkType
 
-    init(networkType: NetworkType) {
-        self.networkType = networkType
-    }
-
     // MARK: Address metadata and validation
     static func getAddressMetadata(_ address: String) -> AddressMetadata? {
         var networkId: UInt32 = 0