- made the helper Node and Iterator classes protected inner classes of List so they don't pollute the android namespace.
- use "int foo()" instead of "int foo(void)" which is more C++ stylish
- made distance() a template function, this way we write it once and it will work with combinations of iterator and const_iterator
- added the inline keyword on some function to make it clear to the compiler and the programmer that we want/intend these to be small inline functions
- added templated comparison operators to Iterator so it can compare iterator and const_iterator
- use size_t instead of "unsigned int" at places
- distance() should return a ptrdiff_t (it's kind of mening less here because it won't really work if the distance is < 0)
- made sure we handle conversions from iterator to const_iterator, but but fail at compile time in the other direction
- added operator->() on iterator and const_iterator
- made a bunch of private constructors explicit to avoid unwanted conversions
