Use withContiguousStorageIfAvailable for String encoding in BinaryEnc…

…oder

Resolves issue #919

Sources/SwiftProtobuf/BinaryEncoder.swift

@@ -39,12 +39,14 @@ internal struct BinaryEncoder {
         }
     }
 
-    private mutating func append(contentsOf bufferPointer: UnsafeRawBufferPointer) {
+    @discardableResult
+    private mutating func append(contentsOf bufferPointer: UnsafeRawBufferPointer) -> Int {
         let count = bufferPointer.count
         if let baseAddress = bufferPointer.baseAddress, count > 0 {
             memcpy(pointer, baseAddress, count)
         }
         pointer = pointer.advanced(by: count)
+        return count
     }
 
     func distance(pointer: UnsafeMutableRawPointer) -> Int {
@@ -125,11 +127,19 @@ internal struct BinaryEncoder {
 
     // Write a string field, including the leading index/tag value.
     mutating func putStringValue(value: String) {
-        let count = value.utf8.count
-        putVarInt(value: count)
-        for b in value.utf8 {
-            pointer.storeBytes(of: b, as: UInt8.self)
-            pointer = pointer.advanced(by: 1)
+        // If the String does not support an internal representation in a form
+        // of contiguous storage, body is not called and nil is returned.
+        let isAvailable = value.utf8.withContiguousStorageIfAvailable { (body: UnsafeBufferPointer<UInt8>) -> Int in
+            putVarInt(value: body.count)
+            return append(contentsOf: UnsafeRawBufferPointer(body))
+        }
+        if isAvailable == nil {
+            let count = value.utf8.count
+            putVarInt(value: count)
+            for b in value.utf8 {
+                pointer.storeBytes(of: b, as: UInt8.self)
+                pointer = pointer.advanced(by: 1)
+            }
         }
     }