[ANN] packetgen 3.2.1 released

packetgen 3.2.1 has been released.

* home: <GitHub - sdaubert/packetgen: Ruby library to easily generate and capture network packets>
* bugs: <Issues · sdaubert/packetgen · GitHub>
* doc: <Home · sdaubert/packetgen Wiki · GitHub>

PacketGen provides simple ways to generate, send and capture network
packets.

Supported protocols are: Ethernet, Dot11, Dot1x, ARP, IP, IPv6, GRE,
ICMP, IGMP, ICMPv6, MLD, MLDv2, OSPFv2, OSPFv3, UDP, TCP, SNMP, DNS,
EAP, BOOTP, DHCP, TFTP, HTTP and MDNS.

Changes:

### 3.2.1 / 2021-12-27

* Add Types::Int#nbits to get size of Int as bits.
* Utils.arp_cache tries using 'ip neigh' command when arp is not installed.
* Fix Utils.arp by ensuring capture is always started before sending ARP request.
* Utils.arp_cache no more crashes when arp utils is not installed. Instead, return an empty cache.
* Utils.arp: ensure ARP request is sent before capturing response.
* Support for Ruby 2.4 is removed.