change API for templated aaLiteral

- let the compiler pass keys and values as slices
  like for the existing _d_assocarrayliteralTX call

- add _aaGetZ which is the same as _aaGetX but also
  get's passed an additional pointer to the typed
  aaLiteral!(Key, Value) function
Merge pull request #1046 from MartinNowak/aaLiteral

change API for templated aaLiteral