Add aggregate operations for stuctures with string fields

Add struct-level comparison and copying of string fields to reduce the
complexity of whole-struct comparison and copying when using string
fields. The new macros do not take into account non-stringfield data.

Review: https://reviewboard.asterisk.org/r/2308/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@381017 f38db490-d61c-443f-a65b-d21fe96a405b