feat(dataclasses): Introduce mlc.dataclasses.stringify

[potatomashed]

With this PR, a dataclass defined in C++ could automatically have a __str__ method that prints it just like Python's dataclass, e.g.

mlc.testing.c_class@0x000103b1bb28(bool_=False, i8=8, i16=16, i32=32, i64=64, f32=1.500000, f64=2.500000, 
raw_ptr=0xdeadbeef, dtype=float8, device=cuda:0, any="hello", func=object.Func@0x000103a7b910, 
ulist=[1, 2.000000, "three", object.Func@0x000103a7b910], 
udict={"2": 2.000000, "4": object.Func@0x000103a7b910, "1": 1, "3": "three"}, 
str_="world", str_readonly="world", list_any=[1, 2.000000, "three", object.Func@0x000103a7b910], 
list_list_int=[[1, 2, 3], [4, 5, 6]], dict_any_any={2.000000: 2, 4: object.Func@0x000103a7b910, 1: 1.000000, "three": "four"}, 
dict_str_any={"4": object.Func@0x000103a7b910, "1": 1.000000, "2.0": 2, "three": "four"}, 
dict_any_str={2.000000: "2", 4: "5", 1: "1.0", "three": "four"}, 
dict_str_list_int={"2": [4, 5, 6], "1": [1, 2, 3]}, opt_bool=True, opt_i64=-64, opt_f64=None, opt_raw_ptr=None, 
opt_dtype=None, opt_device=cuda:0, opt_func=None, opt_ulist=None, opt_udict=None, opt_str=None, opt_list_any=[1, 2.000000, "three", object.Func@0x000103a7b910], 
opt_list_list_int=[[1, 2, 3], [4, 5, 6]], opt_dict_any_any=None, opt_dict_str_any={"4": object.Func@0x000103a7b910, "1": 1.000000, "2.0": 2, "three": "four"}, 
opt_dict_any_str={2.000000: "2", 4: "5", 1: "1.0", "three": "four"}, opt_dict_str_list_int={"2": [4, 5, 6], "1": [1, 2, 3]})

This method can be explicitly invoked via mlc.dataclasses.stringify and should work for both c_class and py_class.