Add documentation for libcc2rs

[lucic71]

While writing the docs for libcc2rs I realized that rc.rs is very heavy. So I took some of the code and put it in other files:

• void.rs - Everything related to AnyPtr. @nunoplopes do you think VoidPtr is a better name?
• ptr_dyn.rs - Everything related to PtrDyn
• cstr.rs - The Ptr<u8> specialization for strings
• move CStringiterator and StringIterator in iterator.rs

I also deleted the unused impl<T> PostfixInc for Cursor<*mut T>

[lucic71]

Merge #302 first