Overall clean-up in maps.h

- Rename some map types:
* IntegerMap -> RangeMap
* StdMap -> SparseMap
* FunctorMap -> FunctorToMap
* MapFunctor -> MapToFunctor
* ForkWriteMap -> ForkMap
* SimpleMap -> WrapMap
* SimpleWriteMap -> WrapWriteMap
- Remove the read-only ForkMap version.
- Rename map-creator functions for the read-write arithmetic and
logical maps.
- Small fixes and improvements in the code.
- Fix the typedefs of RangeMap to work correctly with bool type, too.
- Rename template parameters, function parameters, and private members
in many classes to be uniform and to avoid parameter names starting
with underscore.
- Use Key and Value types instead of K and V template parameters in
public functions.
- Extend the documentation with examples (e.g. for basic arithmetic and
logical maps).
- Many doc improvements.
- Reorder the classes.
- StoreBoolMap, BackInserterBoolMap, FrontInserterBoolMap,
InserterBoolMap, FillBoolMap, SettingOrderBoolMap are almost unchanged,
since they will be removed.
- Also improve maps_test.cc to correctly check every map class, every
constructor, and every creator function.