Change SPI protocol for new feature requests

[chrishamm]

The protocol for data transfers between RRF and DSF needs to be changed again to meet the following requirements:

• Implement file enumerations via SBC (Implement file enumerations via SBC #180)
• Include precision to report for evaluation results (Decimal places in analog readings in SBC mode #187)
• Pass line and column numbers to expression evaluation requests (Pass line and column numbers to expression evaluation requests #195)
• Return file position of the second file being printed (where applicable) when pausing. At present, only the main fpos is returned

These protocol changes should be done in v3.7.