Move ast_str_container_alloc and friends

This moves ast_str_container_alloc, ast_str_container_add,
ast_str_container_remove, and related private functions into
strings.c/h since they really don't belong in astobj2.c/h.

As a result of this move, utils also had to be updated.

Review: https://reviewboard.asterisk.org/r/2719/
(closes issue ASTERISK-22041)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396105 65c4cc65-6c06-0410-ace0-fbb531ad65f3