Loading market data for a script to run over.
The data itself is [pine_core::Data] — bars plus the symbol and timeframe
they belong to. This crate is the ways of getting one: a CSV file today, an
exchange later.
Series history and stateful builtins accumulate as bars execute, so a script must be replayed from its first bar. Handing over the whole series at once is what makes that the only thing a caller can do.