Write value over 8 signed bytes, following 1 metadata byte

Property definitions

msgpack :: write $ Writer :: write_msgpack_int64
	# Write `value` over 8 signed bytes, following 1 metadata byte
	fun write_msgpack_int64(value: Int)
	do
		write_byte 0xD3
		write_int64 value
	end
lib/msgpack/write.nit:126,2--131,4