examine temporary tuple constructions during the runtime

[lemire]

The struct_to_tuple function builds, at runtime, what is effectively a copy of any struct, but organized as a tuple.

So if you have...

struct mytype {
 std::string value1;
 std::string value2;
 int x;
}

Then it will construct (at runtime), something like this...

std::tuple<std::string,std::string,int> t;

That's at runtime, so it is not at all free.

You can help things a little bit by casting to references so that you have...

std::tuple<std::string&,std::string&,int&> t;

In at least one benchmark, just replacing the copies by reference made a large difference (see #14).

Still, we construct an std::tuple for every single struct or class we serialize. While this construction is convenient, it might be unnecessary. A sufficiently advanced compiler can probably lift the overhead, but it might be better if we could get rid of this intermediate construction.

[lemire]

Turns out that it is quite easy to remove the temporary tuples, see commit cd436d2