[C++] Change struct to class for objects that meet the criteria in the Google style guide

[asfimport]

See https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes. I have suspected that the types in type.h should be classes, but this suggests it pretty strongly. It would be better to address this sooner rather than later.

We should also make the member access functions instead of bare attributes, e.g. type->id() instead of type->type

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow 0.3 release (blocks)

_{Note: This issue was originally created as ARROW-782. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #520

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 520
#520