feat: add `struct SimpleDynamicString` definition and two getter functions for it
feat: add `struct SimpleDynamicString` definition and two getter functions for it

 1. SimpleDynamicString structure is used to store string contents.
 2. getter function `get_used` get the member `used_`. O(1)
 3. getter function `get_free` get the member `free_`. O(1)

gaoxiangnumber1/nosql