Implement equality operation for Decimal types

[stinodego]

Reproducible example

import polars as pl
from decimal import Decimal as D

s1 = pl.Series([D("1.00000"), D("2.00000")], dtype=pl.Decimal)
s2 = pl.Series([D("1.00000"), D("2.00001")], dtype=pl.Decimal)

result = s1 == s2
# pyo3_runtime.PanicException: not implemented

Issue description

Decimal equality is not implemented.

Expected behavior

Result should be [True, False]

from polars.testing import assert_series_equal

expected = pl.Series([True, False])
assert_series_equal(result, expected)