arrow

ARROW-1600: [C++] Add Buffer constructor that wraps std::string

Many other libraries interchange binary data with `std::string`. This makes it easy to wrap such data in an `arrow::Buffer`.

It may be worth adding a function that creates a buffer from a string, but owns its memory.

I also deprecated `arrow::GetBufferFromString`, which shouldn't have been public in the first place, since the new ctor is more general

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #1147 from wesm/ARROW-1600 and squashes the following commits:

f60f502c [Wes McKinney] Remove TestBuffer fixture
644bf2b7 [Wes McKinney] Add Buffer ctor that wraps std::string