Better Handling of Variable Length Attributes

[Shelnutt2]

We should add high level functions to handle variable length attributes in queries better. Right now variable length attributes that are being written but have their variable length squashed into a single vector. When reading the user is expect to provide/use a squashed vector with a second vector holding the offsets. It would be a nicer experience if we supported ingesting an slice of go strings, or a slice of slices for other variable length datatypes. In a higher level function the flattening can be handled.

The same idea could be used for reads, attribute buffers can be converted from flat buffers to more usable slices in go.