Add string get function.

- also fixed style errors in string.h and .cpp
Add operators + and - for iterators

- operator +(iterator, sizetype) : calls ++ sizetype times.
- same for -
- shorthands += and -=
fix bug in vector construction

- fix multiple length initializations resultion in improper linking.
- add segfault guard for operator[]
- fix begin() and end() : set the index.