[struct_pack] enhance serialize performance by uninitialized resize

[poor-circle]

Why

In old version, if struct_pack save binary to a container, it will call member function resize(). which cause useless initialization.

What is changing

Add a helper function struct_pack::detail::resize(), which will resize some container(std::basic_string<char/unsigned char/signed char>, std::vector<char/unsigned char/signed char/std::byte>) without initialization.

In benchmark:

old version:

struct_pack serialize 1 rect :            3 ns
struct_pack serialize 20 rects :          17 ns
struct_pack serialize 1 person :          11 ns
struct_pack serialize 20 persons :        110 ns
struct_pack serialize 1 monster :         76 ns
struct_pack serialize 20 monsters :       652 ns

new version:

struct_pack serialize 1 rect :            2 ns
struct_pack serialize 20 rects :          11 ns
struct_pack serialize 1 person :          7 ns
struct_pack serialize 20 persons :        98 ns
struct_pack serialize 1 monster :         76 ns
struct_pack serialize 20 monsters :       618 ns

If the person's name field length is 10000(almost all data are string)
old version:

struct_pack serialize 20 persons :        15226 ns

new version:

struct_pack serialize 20 persons :        9406 ns

So in best case, struct_pack serialization is 1.4x faster than old version if you save your struct to std::vector/std::string

There is no change if you save your struct to stream/raw pointer.