CFFI Manual update

- Update known issues (callback.bff.[12] failures).
- defbitfield, foreign-bitfield-symbols and foreign-bitfield-value.
- Fix typo in tutorial. defenum -> defcenum.
- New section "Other Types" documenting :string, :boolean and
  :wrapper.
- Add note about the translate-* methods not being meant to be called
  directly and suggesting convert-* instead.
- Document the macroexpansion-time translators in a new
  "Optimizing Type Translators" sub-section.
- Document defcenum's base-type option.
- Document defcfun's varargs support.