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
• Skip SPI delays if DSF requested meta G-code as part of the last transaction to speed up the execution
• Include channel in EvaluateExpression and SetVarResult responses
• Add separate notify packet for seq increments instead of M308
• Add separate notify packet for IP address changes
• Add support for Bitmap types to Eval/SetVar responses
• Add new packet to set the G-code result when a SBC code has finished
• Consider sending line number along with codes from RRF to DSF instead of prefixing it as string

These protocol changes should be done in v3.7.