#2 Add Dynamic Array

[Hui Zhou]

Dynamic Array reallocates its size on appending. String is
a subclass of Array, so Dynamic Array will give us
Dynamic String too.

DJB's string implementation or Fefe's libowfat
implementation seem neat, I probably will immitate that.

Or should I just use libowfat? I believe adding the code
verbosely into dragon won't increase the size much nor
the clearness, so I am leaning to copy the implementation
(against link to libowfat).