Revert "Restructure {reasoning_gym, tests}/{core, exercises, curricula}"

This reverts commit 10dbb374b0.

reasoning_gym/arithmetic/__init__.py

@@ -0,0 +1,30 @@
+"""
+Arithmetic tasks for training reasoning capabilities:
+- Basic arithmetic
+- Chain sums
+- Word problems
+- Leg counting
+- Time intervals
+"""
+
+# from .basic_arithmetic import BasicArithmeticDataset
+# from .calendar_arithmetic import CalendarArithmeticDataset
+from .chain_sum import ChainSumDataset
+# from .fraction_simplification import FractionSimplificationDataset
+# from .gcd import GcdDataset
+# from .lcm import LcmDataset
+# from .leg_counting import LegCountingDataset
+# from .prime_factorization import PrimeFactorizationDataset
+# from .time_intervals import TimeIntervalsDataset
+
+__all__ = [
+    # "BasicArithmeticDataset",
+    # "CalendarArithmeticDataset",
+    "ChainSumDataset",
+    # "FractionSimplificationDataset",
+    # "GcdDataset",
+    # "LcmDataset",
+    # "LegCountingDataset",
+    # "PrimeFactorizationDataset",
+    # "TimeIntervalsDataset",
+]