Figure out checksums in System section

[davidlang42]

The system section consists of sub-sections:

• Common
• Compressor
• VLink
• Favorites
• SwitchAssign
  Before each of these is 2 bytes of padding (always "0000000 00000001") and after each is 2 bytes which appears to be a checksum (because it changes when the settings in that section change).

This ticket is to:

• determine how these check sums work (or if they arent check sums, what are they?)
• implement checking the sum in the from_bytes() method and writing the checksum in the to_bytes() method and remove the struct fields so they aren't stored
• update to_structured_json()/from_structured_json() to make a nested collections of these sub-sections (very similar to LiveSet::to/from_structured_json)

[davidlang42]

For what its worth, the RD300NX keyboard seems happy to load an RDS file with incorrect checksums (both the System ones being discussed here and the LiveSet ones which are already working correctly).

[davidlang42]

Still not fully understood, but good enough to confidently read and write system settings. 9534de9