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ARROW-3459: [C++][Gandiva] support for string o/p

- If the output vectors aren't provided, allow resizable
  data buffers.
- If the output vectors are provided, assert that the data
  buffer is resizeable.
- use a cpp function to write to string-like o/p buffers,
  this checks for capacity and updates the offset vector.

Author: Pindikura Ravindra <ravindra@dremio.com>

Closes #4760 from pravindra/varlen and squashes the following commits:

0068b6a07 <Pindikura Ravindra> ARROW-3459:  support for string o/p