Use CUBIC as TCP congestion controller

Support for TCP congestion control has been introduced in smoltcp 0.12.
Smoltcp supports Reno and CUBIC. Of the two CUBIC has higher throughput.
Enabling the socket-tcp-cubic feature is enough to default to CUBIC.

With this change pkg install libgcc is about 35% faster than previously
without congestion control. (5.5MiB/s vs 4MiB/s previously)

Cargo.toml

@@ -38,7 +38,7 @@ features = [
     "std",
     "medium-ethernet", "medium-ip",
     "proto-ipv4",
-    "socket-raw", "socket-icmp", "socket-udp", "socket-tcp",
+    "socket-raw", "socket-icmp", "socket-udp", "socket-tcp", "socket-tcp-cubic",
     "iface-max-addr-count-8",
     "log"
 ]