A surprising amount of time is/was spent comparing keys in the
std::map red/black tree traversing nodes. Since we don't need
the data truly sorted, instead sort in length buckets. Kills
90k rtl_ustring_compare_withLength calls on startup, around
0.9% of headless start.

Change-Id: Ib23aff151ad50d56bbf2ba3e28882cc81898d9ec