make parallel generator movable

Summary:
If `Source` isn't copyable, parallel generator cannot be composed
further due to implicitly deleted move ctor.

Example:

```
byLines(File(...))
| parallel(map(...))
| sum;
```

Reviewed By: ddrcoder, ot

Differential Revision: D10004477

fbshipit-source-id: 51ce23e834413bd53547a683f8ed89a1811f74a2

folly/gen/Parallel-inl.h

@@ -172,11 +172,12 @@ class Parallel : public Operator<Parallel<Ops>> {
                             Composed,
                             Output,
                             OutputDecayed>> {
-    const Source source_;
-    const Ops ops_;
-    const size_t threads_;
-    typedef ClosableMPMCQueue<InputDecayed> InQueue;
-    typedef ClosableMPMCQueue<OutputDecayed> OutQueue;
+    Source source_;
+    Ops ops_;
+    size_t threads_;
+
+    using InQueue = ClosableMPMCQueue<InputDecayed>;
+    using OutQueue = ClosableMPMCQueue<OutputDecayed>;
 
     class Puller : public GenImpl<InputDecayed&&, Puller> {
       InQueue* queue_;