Previously, NMRefString was the public part of the struct, while
there was an internal RefString struct with private fields.
That might make sense if we would need to preserve some stable ABI, but
we don't because this is all internal (unstable) API. It also might
make sense to hide fields, but in practice that is not necessary
because the leading underscore is indicator enough that these are
private fields that are not supposed to be touched (unless you really
know what you do). So, drop RefString and move all fields in the public
NMRefString. The advantage is that we can later inline certain trivial
functions, that we otherwise couldn't.
Also, drop the "str" pointer and only use the "str" array field. The
pointer existed so that during nm_ref_string_new_len() we could create
a lookup needle with external str pointer. That is now solved
differently by using "len == G_MAXSIZE" as indicator that this is
a special lookup instance. The advantage is that we save one pointer
field per NMRefString, that we reduce the redundancy of the data, and
that we don't need the additional indirection.
