[WIP] Support multiple instruments(symbols) backtest

[robert1003]

Fixes #20

An incomplete attempt to add multiple-symbol backtest, which is mentioned in #20. Not sure if this fits the current structure.

Lots of necessary changes haven't been done yet, including docs and code related graphing part.

[robert1003]

How do you think @kernc? If you think it is inappropriate I will withdraw it, thanks.

[AgarwalPragy]

It looks like the current attempt is not backwards compatible. For example, the signature of Strategy.position has changed from Position to dict[str, Position]. 😞

Maybe we can include a flag which enables multi-symbol mode and return Union[Position, dict[str, Position]] based on whether the mode is enabled or not?

[AgarwalPragy]

There is also something wrong with Broker.last_price and other functions.

[AgarwalPragy]

I've made a parallel attempt here #641

[luketych]

How is this coming along?

[PabloCanovas]

Not related in anyway with him but @dodid did an awesome job extending this backtesting.py library to his minitrade package which enables multiasset backtesting, check it out: https://github.com/dodid/minitrade