ENH: add point in time aspect to equity symbol mapping

Changes the overlap behavior so that it is an error to write data which
would have two companies holding the same ticker. Other than one test
around which company would win in that case, all the other tests are
passing. That single test has been changed to check the write-time
error.

tests/test_algorithm.py

@@ -777,7 +777,10 @@ class TestTransformAlgorithm(WithLogger,
 
     @classmethod
     def make_futures_info(cls):
-        return pd.DataFrame.from_dict({3: {'multiplier': 10}}, 'index')
+        return pd.DataFrame.from_dict(
+            {3: {'multiplier': 10, 'symbol': 'F'}},
+            orient='index',
+        )
 
     @classmethod
     def make_equity_daily_bar_data(cls):
@@ -985,6 +988,7 @@ def before_trading_start(context, data):
             'start_date': start_session,
             'end_date': period_end + timedelta(days=1)
         }] * 2)
+        equities['symbol'] = ['A', 'B']
         with TempDirectory() as tempdir, \
                 tmp_trading_env(equities=equities) as env:
             sim_params = SimulationParameters(
@@ -2813,6 +2817,7 @@ class TestTradingControls(WithSimParams, WithDataPortal, ZiplineTestCase):
         metadata = pd.DataFrame.from_dict(
             {
                 1: {
+                    'symbol': 'SYM',
                     'start_date': start,
                     'end_date': start + timedelta(days=6)
                 },
@@ -2940,6 +2945,7 @@ class TestTradingControls(WithSimParams, WithDataPortal, ZiplineTestCase):
 
     def test_asset_date_bounds(self):
         metadata = pd.DataFrame([{
+            'symbol': 'SYM',
             'start_date': self.sim_params.start_session,
             'end_date': '2020-01-01',
         }])
@@ -2959,6 +2965,7 @@ class TestTradingControls(WithSimParams, WithDataPortal, ZiplineTestCase):
             algo.run(data_portal)
 
         metadata = pd.DataFrame([{
+            'symbol': 'SYM',
             'start_date': '1989-01-01',
             'end_date': '1990-01-01',
         }])
@@ -2979,6 +2986,7 @@ class TestTradingControls(WithSimParams, WithDataPortal, ZiplineTestCase):
                 algo.run(data_portal)
 
         metadata = pd.DataFrame([{
+            'symbol': 'SYM',
             'start_date': '2020-01-01',
             'end_date': '2021-01-01',
         }])