[struct_pack][breakchange] use struct_pack::err_code instead of struct_pack::errc as return value

[poor-circle]

Why

Closes #284

Now the return type of error code is a enum class. Which is hard to use when error handling.

struct_pack::errc ec = struct_pack::deserialize_to(...);
if (ec == struct_pack::errc{})
{
   // ok branch
}
else 
{
  // error branch
}

What is changing

The type of error now is changed to struct struct_pack::err_code(contain a value of struct_pack::errc) not the enum class struct_pack::errc.

Now we can simply the error handling.

struct_pack::err_code ec = struct_pack::deserialize_to(...);
if (!ec)
{
   // ok branch
}
else 
{
  // error branch
}

It's easy for user to update, the old error-handling code-style is still work. and we dont remove the enum class struct_pack::err_code. It's easy to convert each other.

Example

See document.