Change visibility of folly::Arena constructors to public

Summary: In the case of the copy constructor, this change
doesn't really matter, since it's deleted anyway.

In the case of the move constructor, it fixes what was probably
an unintentional hiding.  The class certainly looks like
it supports moving.

Closes #121

Reviewed By: @yfeldblum

Differential Revision: D2184131

folly/Arena.h

@@ -119,7 +119,6 @@ class Arena {
     return bytesUsed_;
   }
 
- private:
   // not copyable
   Arena(const Arena&) = delete;
   Arena& operator=(const Arena&) = delete;
@@ -128,6 +127,7 @@ class Arena {
   Arena(Arena&&) = default;
   Arena& operator=(Arena&&) = default;
 
+ private:
   struct Block;
   typedef boost::intrusive::slist_member_hook<
     boost::intrusive::tag<Arena>> BlockLink;

folly/test/ArenaTest.cpp

@@ -157,6 +157,17 @@ TEST(Arena, SizeLimit) {
   EXPECT_THROW(arena.allocate(maxSize + 1), std::bad_alloc);
 }
 
+TEST(Arena, MoveArena) {
+  SysArena arena(sizeof(size_t) * 2);
+  arena.allocate(sizeof(size_t));
+  auto totalSize = arena.totalSize();
+  auto bytesUsed = arena.bytesUsed();
+
+  SysArena moved(std::move(arena));
+  EXPECT_EQ(totalSize, moved.totalSize());
+  EXPECT_EQ(bytesUsed, moved.bytesUsed());
+}
+
 int main(int argc, char *argv[]) {
   testing::InitGoogleTest(&argc, argv);
   gflags::ParseCommandLineFlags(&argc, &argv, true);