stringfields: Update extended string fields for master only.

In 13, the new ast_string_field_header structure had to be dynamically
allocated and assigned to a pointer in ast_string_field_mgr to preserve ABI
compatability.  In master, it can be converted to being a structure-in-place in
ast_string_field_mgr to eliminate the extra alloc and free calls.

Change-Id: Ia97c5345eec68717a15dc16fe2e6746ff2a926f4