Core: move digest-related functions from lib/common/xml.c to new digest.c

Specifically:
dump_xml_for_digest()
calculate_xml_digest_v1()
calculate_xml_digest_v2()
calculate_on_disk_digest()
calculate_operation_digest()
calculate_xml_versioned_digest()

Because this required that dump_xml() and fix_digest_buffer() be non-static,
those were renamed to crm_xml_dump() and crm_buffer_add_char() respectively
and declared in crm_internal.h.