gnrc_sixlowpan_frag: factor-out and rename fragmentation buffer

[miri64]

Contribution description

The name fragment_msg or frag_msg/msg_frag always to me was a bit misplaced, as it basically implements an asynchronous fragmentation buffer and doesn't necessarily have anything to do with messages. This change

1. changes the name to fb (for fragmentation buffer)
2. factors its code out to its own sub-module so it can be re-used by other 6LoWPAN fragmentation schemes like Selective Fragment Recovery

Testing procedure

make -C tests/gnrc_sixlowpan_frag flash test

still works (I tested on native and samr21-xpro) and pinging large (>100 bytes) packets between two nodes using 6LoWPAN (e.g. iotlab-m3) with examples/gnrc_networking still works and shows comparable performance to master (tested in the IoT-LAB in my case).

In this PR:

1571913323.341546;m3-104;> ping6 -s 1232 fe80::1711:6b10:65f9:5d0a
1571913323.498874;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=0 ttl=64 rssi=-46 dBm time=154.238 ms
1571913324.508768;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=1 ttl=64 rssi=-46 dBm time=164.468 ms
1571913325.520475;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=2 ttl=64 rssi=-46 dBm time=176.166 ms1571913549.059693;

On master:

m3-104;ping6 -s 1232 fe80::1711:6b10:65f9:5d0a
1571913546.881919;m3-104;> ping6 -s 1232 fe80::1711:6b10:65f9:5d0a
1571913547.038380;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=0 ttl=64 rssi=-46 dBm time=154.214 ms
1571913548.039440;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=1 ttl=64 rssi=-46 dBm time=155.460 ms
1571913549.059439;m3-104;1240 bytes from fe80::1711:6b10:65f9:5d0a: icmp_seq=2 ttl=64 rssi=-46 dBm time=177.131 ms

Issues/PRs references

None

[miri64]

Rebased to current master.

[miri64]

Rebased to current master

[benpicco]

This really just renames and moves code around.
No functional changes expected.