Freeciv

Introduce a new dataio type "arraylen" that serves both to encode the length of an array, and to create that array in JSON mode.

• In binary mode, this is just read and written as an appropriate integer type (uint16 should do unless maximum packet length gets a lot bigger)
• In JSON mode, this is written by creating an array, and read by getting the length of that array, without needing extra space (and structure) to transmit the length separately.