v0.19.1

Fixed — a command's checksums are computed after its script

• When a command has a script, every {checksum:...} of its frame is
  computed after the script has run, over the bytes the script leaves: a
  function byte the script writes before a checksum is covered. The script
  sees 0x00 where a checksum goes, and the engine overwrites what it writes
  there. A script that changes the frame's length on a command holding a
  checksum is refused (nothing is sent, the error names the driver and the
  command). Commands without a script build as in 0.19.0.