MAINT: Move equity data formats out of loader.

Put the logic for reading and writing the equity price and adjustment
data into a module located in data, making it distinct from the pipeline
loader usage of the formats.

This prepares for both incoming changes of how adjustments are written,
(which includes using the bcolz daily reader as an input), as well as
eventually providing the readers to a DataPortal object.

tests/pipeline/test_engine.py

@@ -30,12 +30,12 @@ from zipline.pipeline.loaders.synthetic import (
     NullAdjustmentReader,
     SyntheticDailyBarWriter,
 )
+from zipline.data.us_equity_pricing import BcolzDailyBarReader
 from zipline.finance.trading import TradingEnvironment
 from zipline.pipeline import Pipeline
 from zipline.pipeline.data import USEquityPricing
 from zipline.pipeline.loaders.frame import DataFrameLoader, MULTIPLY
 from zipline.pipeline.loaders.equity_pricing_loader import (
-    BcolzDailyBarReader,
     USEquityPricingLoader,
 )
 from zipline.pipeline.engine import SimplePipelineEngine