Expose a human-readable type name for a dynamic

Summary: Aids in debugging dynamic types, etc.

Test Plan: unit tests

Reviewed By: delong.j@fb.com

FB internal diff: D812385

folly/dynamic.cpp

@@ -34,6 +34,10 @@ DEF_TYPE(dynamic::ObjectImpl, "object",  dynamic::OBJECT);
 
 #undef DEF_TYPE
 
+const char* dynamic::typeName() const {
+  return typeName(type_);
+}
+
 //////////////////////////////////////////////////////////////////////
 
 }

folly/dynamic.h

@@ -253,6 +253,11 @@ public:
    */
   Type type() const;
 
+  /*
+   * Returns the type of this dynamic as a printable string.
+   */
+  const char* typeName() const;
+
   /*
    * Extract a value while trying to convert to the specified type.
    * Throws exceptions if we cannot convert from the real type to the