Modernize memcpy() and memmove() usage.

This CL replaces memcpy() with std::memcpy()
and memmove() with std::memmove(), and #includes
<cstring> in files that use either function.

PiperOrigin-RevId: 306067788

snappy_unittest.cc

@@ -84,7 +84,7 @@ class DataEndingAtUnreadablePage {
     CHECK_NE(MAP_FAILED, mem_);
     protected_page_ = reinterpret_cast<char*>(mem_) + space_for_string;
     char* dst = protected_page_ - size;
-    memcpy(dst, s.data(), size);
+    std::memcpy(dst, s.data(), size);
     data_ = dst;
     size_ = size;
     // Make guard page unreadable.