[C++] Improve buffer symbolic index

[asfimport]

The array data buffers vector is index differently depending on the Array type. This feature would expose static constexpr named variables for buffer index.

Reporter: Francois Saint-Jacques / @fsaintjacques

_{Note: This issue was originally created as ARROW-4729. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou: Can you detail what you mean here exactly?

[asfimport]

Wes McKinney / @wesm: probably

static constexpr int kBufferValidity = 0;
static constexpr int kBufferFixedSizeData = 1;
static constexpr int kBufferVarOffsets = 1;
static constexpr int kBufferVarData = 2;

etc

[asfimport]

Francois Saint-Jacques / @fsaintjacques: Precisely

[asfimport]

Antoine Pitrou / @pitrou: Pretty minor, but could be nice to have in 1.0.0.

[asfimport]

Antoine Pitrou / @pitrou: This is probably still nice to have. Should probably be members of the respective DataType subclasses?

[github-actions[bot]]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.