[feat] support user-defined method to serialize/deserialize struct

[poor-circle]

example:

namespace my_space {
struct my_struct {
  int x, y, z;
  bool operator==(const my_struct& o) const {
    return x == o.x && y == o.y && z == o.z;
  }
};

template <bool Is_writing_escape, typename Stream>
inline void to_json_impl(Stream& s, const my_struct& t) {
  iguana::to_json<Is_writing_escape>(*(int(*)[3]) & t, s);
}

template <typename It>
IGUANA_INLINE void from_json_impl(my_struct& value, It&& it, It&& end) {
  iguana::from_json(*(int(*)[3]) & value, it, end);
}

}  // namespace my_space

struct nest {
  std::string name;
  my_space::my_struct value;
  bool operator==(const nest& o) const {
    return name == o.name && value == o.value;
  }
};

REFLECTION(nest, name, value);

void user_defined_struct_example() {
  {
    my_space::my_struct v{1, 2, 3}, v2;
    std::string s;
    iguana::to_json(v, s);
    std::cout << s << std::endl;
    iguana::from_json(v2, s);
    assert(v == v2);
  }
  {
    nest v{"Hi", {1, 2, 3}}, v2;
    std::string s;
    iguana::to_json(v, s);
    std::cout << s << std::endl;
    iguana::from_json(v2, s);
    assert(v == v2);
  }
}

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 21.56863% with 40 lines in your changes are missing coverage. Please review.

Project coverage is 44.67%. Comparing base (b00ba10) to head (ff76e0d).

Files	Patch %	Lines
test/test.cpp	0.00%	21 Missing ⚠️
example/json_example.cpp	0.00%	16 Missing ⚠️
example/xml_example.cpp	0.00%	2 Missing ⚠️
iguana/json_writer.hpp	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #243      +/-   ##
==========================================
- Coverage   44.91%   44.67%   -0.25%     
==========================================
  Files          46       46              
  Lines        5553     5592      +39     
==========================================
+ Hits         2494     2498       +4     
- Misses       3059     3094      +35     

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