util: implement prevector's move ctor & move assignment

Using swap() was rather wasteful because it had to copy the whole direct
memory data twice. Also, due to the swap() in move assignment the moved-from
object might hold on to unused memory for longer than necessary.

Author: martinus

src/prevector.h

@@ -264,8 +264,10 @@ class prevector {
         fill(item_ptr(0), other.begin(),  other.end());
     }
 
-    prevector(prevector<N, T, Size, Diff>&& other) noexcept {
-        swap(other);
+    prevector(prevector<N, T, Size, Diff>&& other) noexcept
+        : _union(std::move(other._union)), _size(other._size)
+    {
+        other._size = 0;
     }
 
     prevector& operator=(const prevector<N, T, Size, Diff>& other) {
@@ -277,7 +279,12 @@ class prevector {
     }
 
     prevector& operator=(prevector<N, T, Size, Diff>&& other) noexcept {
-        swap(other);
+        if (!is_direct()) {
+            free(_union.indirect_contents.indirect);
+        }
+        _union = std::move(other._union);
+        _size = other._size;
+        other._size = 0;
         return *this;
     }