Fix a memory leak in DispatchData.withUnsafeBytes

`DispatchData.withUnsafeBytes` created a new `dispatch_data_t` by calling `dispatch_data_create_map`. I assume that the intention was that this memory was freed when `data` is destroyed, based on the presence of `_fixLifetime(data)` but `data` was just a plain `dispatch_data_t` C struct, that doesn’t have any cleanup operations associated with it when destroyed.

To fix the leak, wrap the `dispatch_data_t` in a `DispatchData`, which takes over the ownership of the `dispatch_data_t` and releases it when `data` gets destroyed.

Alternatively, `_fixLifetime` could have been replaced by `_swift_dispatch_release(unsafeBitCast(data, to: dispatch_object_t.self))` but I think using `DispatchData` is the cleaner solution.

Author: ahoppen

src/swift/Data.swift

@@ -110,7 +110,7 @@ public struct DispatchData : RandomAccessCollection {
 	{
 		var ptr: UnsafeRawPointer? = nil
 		var size = 0
-		let data = CDispatch.dispatch_data_create_map(__wrapped.__wrapped, &ptr, &size)
+		let data = DispatchData(data: CDispatch.dispatch_data_create_map(__wrapped.__wrapped, &ptr, &size))
 		let contentPtr = ptr!.bindMemory(
 			to: ContentType.self, capacity: size / MemoryLayout<ContentType>.stride)
 		defer { _fixLifetime(data) }