[C++] Add FixedWidthBinary type

[asfimport]

This would be very useful for interacting with Parquet and other data sources that use fixed-byte-width binary types, as a more efficient version of List. We can resolve adding this type to the format spec later.

@julienledem @nongli do you have an opinion about this type? Would be equivalent to FIXED_LEN_BYTE_ARRAY in Parquet. It would be convenient to implement Decimal types on top of this

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

Related issues:

• [Python] Add support for FixedWidthBinary type (relates to)

_{Note: This issue was originally created as ARROW-491. Please see the migration documentation for further details.}

[asfimport]

Nong Li / @nongli:
+1 I think this is one of the most useful types.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 379
#379

[asfimport]

Julien Le Dem / @julienledem:
So, from the layout perspective, there's just one vector with fixed width values of byteWidth and a validityVector .
Sounds good to me.

[asfimport]

Wes McKinney / @wesm:
Yep, here is the type layout in C++: https://github.com/apache/arrow/blob/master/cpp/src/arrow/type.cc#L303