Add tests for btree and fix a couple issues.

examples/btree.cc

@@ -0,0 +1,30 @@
+#include <iostream>
+#include <string>
+#include <parallel_hashmap/btree.h>
+
+using phmap::btree_map;
+using phmap::btree_set;
+
+int main()
+{ 
+    btree_map<std::string, int> persons = 
+        { { "John", 35 },
+          { "Jane", 32 },
+          { "Joe",  30 },
+        };
+
+    for (auto& p: persons)
+        std::cout << p.first <<  " (" << p.second << ")" << '\n';
+
+    // Create a btree_set of three floats (that map to strings)
+    using X = std::tuple<float, std::string>;
+    btree_set<X> email;
+ 
+    // Iterate and print keys and values 
+    for (int i=0; i<10; ++i)
+        email.insert(X((float)i, "aha"));
+    
+    for (auto& e: email)
+        std::cout << std::get<0>(e) << ", " << std::get<1>(e) << '\n';
+            return 0;
+}