Completely align the API of `BigDecimal` with Python's `Decimal`

[forfudan]

Currently, many methods in Python's Decimal type is not yet implemented in decimo.BigDecimal type. It would be nice that we implement all of them so that Pythonistas can smoothly call these methods.

Below is a summary of all methods of Python's Decimal and whether they are implemented in decimo.BigDecimal. Some of them have already been implemented but with different names, e.g., BigDecimal.exponent() is just Python's decimal.adjusted.

✓ __abs__          ✓ __add__          ✓ __bool__         ✓ __ceil__
✗ __complex__      ✓ __eq__           ✓ __float__        ✓ __floor__
✓ __floordiv__     ✓ __ge__           ✓ __gt__           ✗ __hash__
✓ __int__          ✓ __le__           ✓ __lt__           ✓ __mod__
✓ __mul__          ✓ __ne__           ✓ __neg__          ✓ __pos__
✓ __pow__          ✓ __radd__         ✓ __repr__         ✓ __rfloordiv__
✓ __rmod__         ✓ __rmul__         ✓ __round__        ✓ __rpow__
✓ __rsub__         ✓ __rtruediv__     ✓ __str__          ✓ __sub__
✓ __truediv__      ✓ __trunc__
✓ adjusted         ✗ as_integer_ratio ✓ as_tuple         ✗ canonical
✓ compare          ✗ conjugate        ✗ copy_abs         ✗ copy_negate
✗ copy_sign        ✓ exp              ✗ fma              ✗ is_canonical
✗ is_finite        ✓ is_integer       ✗ is_nan           ✗ is_normal
✗ is_signed        ✗ is_snan          ✗ is_subnormal     ✗ is_qnan
✓ ln               ✓ log10            ✗ logb             ✗ logical_and
✗ logical_invert   ✗ logical_or       ✗ logical_xor      ✓ max
✗ max_mag          ✓ min              ✗ min_mag          ✗ next_minus
✗ next_plus        ✗ next_toward      ✓ normalize        ✗ number_class
✓ quantize         ✗ radix            ✗ remainder_near   ✗ rotate
✗ same_quantum     ✗ scaleb           ✗ shift            ✓ sqrt
✓ to_eng_string    ✗ to_integral_exact ✗ to_integral_value