Add keys function to Index and it's instances

 - ContextIndex
 - DmPrefixTree
 - ComprOccPrefixTree
 - InvertedIndex
   - performance may be increased by changing IKeys
     to String and skip packing all keys
Add TextIndex constraint type
Adjust Processor to the new (Text)Index structure

 - not all of the monadic functions are converted,
   only the exported ones (shallow)
Minor refactoring
Cleanup