feat(dataclass): Support frozen=True in py_class

[potatomashed]

This PR brings support for frozen=True so that one could create dataclass instances with immutability. For example,

import mlc.dataclasses as mlcd

@mlcd.py_class(frozen=True)
class Frozen(mlcd.PyClass):
    a: int
    b: str

x = Frozen(a=1, b="b")

>>> x.a = 2  # not allowed to set a field
AttributeError: property 'a' of 'Frozen' object has no setter

And if one has to forcefully set attributes, there's a private API:

>>> x._mlc_setattr("a", 3)
>>> x.a
3

This design is consistent with python's native dataclass, where it uses object.__setattr__ to force-set fields

[potatomashed]

Should fix #50