Improve code generation

[mingxwa]

Avoid generating duplicated code for trivially-copyable, -relocatable, -destructible pointer types. Although link-time optimizations may help, it is always more efficient to avoid duplication from the front-end.

Before (link):

After (link)

We can also observe significant performance boost at link-time. I tried to compile the following code on my dev machine with MSVC (with linker option /OPT:ICF to avoid generating duplicated code):

#include <proxy/proxy.h>

namespace spec {

PRO_DEF_FACADE(Any, std::tuple<>, pro::copyable_ptr_constraints);

}  // namespace spec

void DoSomethingWithAny(pro::proxy<spec::Any>);

template <std::size_t... Is>
void TestImpl(std::index_sequence<Is...>) {
  (DoSomethingWithAny(pro::make_proxy<spec::Any, std::integral_constant<std::size_t, Is>>()), ...);
}
template <std::size_t N>
void Test() {
  TestImpl(std::make_index_sequence<N>{});
}

int main() {
  Test<1000>();
}

Before this change, compiling it took 6.0 seconds; linking it took with an empty implementation of DoSomethingWithAny() took 7.3 seconds. After this change, compiling it took almost the same time, but linking it with the same implementation of DoSomethingWithAny() only took 1.7 seconds.

Closes #82