[C++] Let StringBuilder be constructible with a pre allocated buffer for character data

[asfimport]

This is useful for example when an existing buffer can be immediately reused. This is currently used for storage of strings in json parsing, so it'd be straightforward to refactor into a constructor of StringBuilder.

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-4698. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou: Do you still need this?

Another possibility would be to implement a caching memory pool (ARROW-3406).

[asfimport]

Wes McKinney / @wesm: This seems hypothetically useful to avoid an extra copy step if you have a partially-constructed MutableBuffer that you wish to continue appending to. The construction requirements are a bit onerous though, since the internal state of the builder needs to be exactly right

[asfimport]

Antoine Pitrou / @pitrou: The first step here may be to add this functionality to BufferBuilder / TypedBufferBuilder.

[github-actions[bot]]

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