Make CScript (and prevector) c++11 movable.

Such moves are used when reallocating vectors that contain them,
for example.

src/prevector.h

@@ -248,6 +248,10 @@ public:
         }
     }
 
+    prevector(prevector<N, T, Size, Diff>&& other) : _size(0) {
+        swap(other);
+    }
+
     prevector& operator=(const prevector<N, T, Size, Diff>& other) {
         if (&other == this) {
             return *this;
@@ -263,6 +267,11 @@ public:
         return *this;
     }
 
+    prevector& operator=(prevector<N, T, Size, Diff>&& other) {
+        swap(other);
+        return *this;
+    }
+
     size_type size() const {
         return is_direct() ? _size : _size - N - 1;
     }