fvec: add append_range() function

This allows one to easily append multiple elements to the end at
once.

core/src/stored/backends/dedup/fvec.h

@@ -32,6 +32,7 @@ extern "C" {
 }
 #include <system_error>
 #include <limits>
+#include <cstring>
 
 namespace dedup {
 struct access {
@@ -151,6 +152,14 @@ template <typename T> class fvec : access {
     }
   }
 
+  // think of (arr, size) as a span; then the name makes sense
+  void append_range(const T* arr, std::size_t size)
+  {
+    reserve_extra(size);
+    std::memcpy(end(), arr, size * element_size);
+    count += size;
+  }
+
   void reserve_extra(size_type additional) { reserve(additional + count); }
 
   void clear() { count = 0; }