warnings: Compiler warning on memset usage for non-trivial type

Problem:
- IS_TRIVIALLY_CONSTRUCTIBLE macro does not work correctly resulting
  in `memset()` usage to set a non-trivial type to 0 when
  `nontrivial_t` is passed in from the tests.
- Warning reported by GCC when compiling with `--enable-werror`.

Solution:
- Use the standard algorithm `std::fill_n()` and let the compiler
  determine the optimal way of looping or using `memset()`.

src/prevector.h

@@ -10,6 +10,8 @@
 #include <stdint.h>
 #include <string.h>
 
+#include <algorithm>
+#include <cstddef>
 #include <iterator>
 #include <type_traits>