Support `DataInput` for parsing

[cowtowncoder]

Since some frameworks (like Hadoop) make heavy use of DataInput and DataOutput (via Hadoop Writable), it'd be nice to support them. Whereas DataOutput support was easy (see #276), DataInput is more involved, but seems doable. It does, however, have to implemented separately for all backends, unlike DataOutput.

[cowtowncoder]

Implemented for basic JSON parser; support for other formats must be added separately: most likely will try Smile, CBOR implementations in near future.