glz::for_each_field

[stephenberry]

Adds glz::for_each_field for reflected and meta types.

struct test_type {
   int32_t int1{};
   int64_t int2{};
};

suite reflect_test_type = [] {
   static_assert(glz::reflect<test_type>::size == 2);
   static_assert(glz::reflect<test_type>::keys[0] == "int1");
   
   "for_each_field"_test = [] {
      test_type var{42, 43};
      
      glz::for_each_field(var, [](auto& field) {
          field += 1;
      });
      
      expect(var.int1 == 43);
      expect(var.int2 == 44);
   };
};