Add hwloc_obj_type_sscanf() to extend hwloc_obj_type_of_string()

The former was already used in hwloc-calc.h to parse things like
"L2iCache" or "Group3". Make it officially part of the API and
deprecate the old hwloc_obj_type_of_string().
synthetic: add support for L2cache, L1i, group3, etc for for input and export
synthetic: rework the storage of synthetic info in the backend private data