Future proof the instrument system

[stijnfrishert]

• Don't immediately parse instruments, just store their bytes and the format version
  • Storing the version allows us to reject/migrate instruments moving between songs of different formats.
• Provide accessor functions to read data from an instrument.
  • These should be in the form bool lsdj_instrument_read_envelope(instr, char** envelope)
  • The fmt version can be used to branch
  • The result is put in an output parameter, which we can also fill with null if said setting doesn't exist for a given version
  • The function returns success/failure for easy if-statements