include/asterisk/vector.h: Backport vector.h to Asterisk 11

Vectors are very useful constructs. As a container, they prevent having
to calloc/realloc arrays manually. They also have advantages over linked
lists, which require elements in the list to be a struct. This patch
backports vectors to Asterisk 11 for use in future patches.

Change-Id: Idc9d74d246a0158b0b36ccb250e7acc71bab078d