manual: convert-* interface, optimizing translators overview optimized
manual: convert-* interface, optimizing translators overview optimized

- (Wrapper generators): Add comment about the 30-90% figure.
- Reword `Pointers' to be shorter/clearer.
- Promote `Optimizing Type Translators' to a full section, a few other
  things here, most conspicuously expanding on the note about expand-*
  method definition time.
- (Tutorial-Conclusion): Not a first draft anymore.
- Document convert-to-foreign, convert-from-foreign, and
  free-converted-object.

marsijanin/cffi,kingcheez/cffi-cheez,attila-lendvai/cffi,attila-lendvai/cffi,cffi/cffi,dochang/cffi,stassats/cffi,cffi/cffi,mcna/cffi,dkochmanski/cffi,3b/cffi,dkochmanski/cffi,kingcheez/cffi-cheez,mcna/cffi,stassats/cffi,dochang/cffi,wesen/cffi,wesen/cffi,cffi/cffi,3b/cffi,stassats/cffi,marsijanin/cffi,akovalenko/cffi,kingcheez/cffi-cheez,3b/cffi,marsijanin/cffi,dkochmanski/cffi,akovalenko/cffi,dochang/cffi,wesen/cffi,akovalenko/cffi