set the allowed max depth of nested blocks to 4

This was needed since I used 'for ... in' loops with hasOwnProperty
checks which would be rejected otherwise.
Some fixes and comments on PrefixMappingImpl

- rewrote forEach loops to for-in loops since forEach didn't work for
  simple prefix maps
- fixed the shortForm method
- added some TODO comments
- minor changes (removed comma, ...)
added tests for PrefixMappingImpl