added a core implementation of FIB tables using generic addresses in an outer table structure
- the basic functions are implemented for operating FIB tables, register RPs/RRPs
  and signal RRPs when no destination is available for a next hop request
- generic addresses are implemented using a use_count to trigger if an address can be reused/overwritten
  once allocated memory will remain allocated until calling deintialisation for `generic_address`, pruning all entries
  or when an unsed entry is overwritten with a new entry of distinct size
- FIB tables are statically represented by an array of entries pointing to generic_address entries

TODO:
- handling for preference sets for colliding next hops
- test the implementation with the real network layer
- using the type field to recognize/apply compression
  - use compression, e.g. LOWPAN_IPHC

NOTE:
this is a prototype implementation it can be considered just as a proof of concept in this state
It follows strictly KISS. So, no fancy algorythms nor code hocus-pocus is applied.