doc/core-data-structures.md: Describe packet/link/app
doc/core-data-structures.md: Describe packet/link/app

This new document explains the three core Snabb data structure:
packet, link, and app. For now this is a stand-alone markdown file but
it could be incorporated into the introduction in the manual.

This is imported from lukego/blog#11.

SnabbCo/snabbswitch,Igalia/snabb,snabbco/snabb,heryii/snabb,Igalia/snabb,snabbco/snabb,eugeneia/snabb,Igalia/snabb,eugeneia/snabb,Igalia/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,kbara/snabb,snabbco/snabb,heryii/snabb,dpino/snabb,dpino/snabbswitch,dpino/snabb,eugeneia/snabb,heryii/snabb,eugeneia/snabb,kbara/snabb,eugeneia/snabbswitch,wingo/snabb,Igalia/snabb,eugeneia/snabb,alexandergall/snabbswitch,Igalia/snabbswitch,wingo/snabbswitch,snabbco/snabb,Igalia/snabbswitch,dpino/snabb,kbara/snabb,Igalia/snabb,alexandergall/snabbswitch,snabbco/snabb,Igalia/snabbswitch,wingo/snabb,mixflowtech/logsensor,wingo/snabb,Igalia/snabb,SnabbCo/snabbswitch,Igalia/snabbswitch,wingo/snabbswitch,mixflowtech/logsensor,wingo/snabb,dpino/snabb,heryii/snabb,mixflowtech/logsensor,wingo/snabbswitch,dpino/snabbswitch,dpino/snabbswitch,dpino/snabb,eugeneia/snabb,wingo/snabb,heryii/snabb,kbara/snabb,snabbco/snabb,snabbco/snabb,snabbco/snabb,mixflowtech/logsensor,wingo/snabb,eugeneia/snabbswitch,dpino/snabbswitch,dpino/snabb,eugeneia/snabb,dpino/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch,heryii/snabb,kbara/snabb,mixflowtech/logsensor,alexandergall/snabbswitch,eugeneia/snabb,Igalia/snabbswitch,wingo/snabbswitch,Igalia/snabb,eugeneia/snabbswitch,alexandergall/snabbswitch,eugeneia/snabbswitch,kbara/snabb,alexandergall/snabbswitch,SnabbCo/snabbswitch