memusage: Add DynamicUsage for std::string

Add DynamicUsage(std::string) which Returns the dynamic allocation of a std::string,
or 0 if none (in case of small string optimization).

src/memusage.h

@@ -15,6 +15,7 @@
 #include <map>
 #include <memory>
 #include <set>
+#include <string>
 #include <vector>
 #include <unordered_map>
 #include <unordered_set>
@@ -90,6 +91,18 @@ static inline size_t DynamicUsage(const std::vector<T, Allocator>& v)
     return MallocUsage(v.capacity() * sizeof(T));
 }
 
+static inline size_t DynamicUsage(const std::string& s)
+{
+    const char* s_ptr = reinterpret_cast<const char*>(&s);
+    // Don't count the dynamic memory used for string, if it resides in the
+    // "small string" optimization area (which stores data inside the object itself, up to some
+    // size; 15 bytes in modern libstdc++).
+    if (!std::less{}(s.data(), s_ptr) && !std::greater{}(s.data() + s.size(), s_ptr + sizeof(s))) {
+        return 0;
+    }
+    return MallocUsage(s.capacity());
+}
+
 template<unsigned int N, typename X, typename S, typename D>
 static inline size_t DynamicUsage(const prevector<N, X, S, D>& v)
 {