First cut of a libsolv-style string <-> id pool API

First cut of a libsolv-style string <-> id pool API

- The pool stores "arbitrary" number of strings in a space-efficient
  manner, with near constant (hashed) string -> id lookup/store and
  constant time id -> string and id -> string length lookups.
- Credits for the idea go to the Suse developers working on libsolv,
  the basic concept is directly lifted from there but details
  differ due to using rpm's own hash table implementation etc.
  Another minor difference is using size_t for offsets to permit over
  4GB total data size on 64bit systems, the total number of id's in
  the pool is limited to uint32 max however (like in libsolv).
- Any (re)implementation bugs by yours truly, this is almost certainly
  going to need further tuning and tweaking, API and otherwise.