Array Datatype API and Operations

[cpcloud]

We should enable basic operations on ARRAY<T> types after #848 is merged.

• array.length(): number of elements in each array in a column
• array[start:stop] and array[index], i.e., __getitem__: element selection
• array.collect: collecting scalar values into an GROUP BY
• array.contains(element): is element in array
• array.concat(*arrays): concatenate array with *arrays
• array.find(element): returns the index of the first occurrence of element and -1 if it's not found

Most of the work that needs to be done here is in the type system.

@wesm I've done a lot of this work, but in retrospect I think renaming Array-the-column to Field is a better in the long term to disambiguate it from Array-the-element-type. Otherwise we get things like ArrayArray (as opposed to ArrayField).

xref #782

[wesm]

Yeah, I think either Field or Column is a reasonable renaming -- one question is that there are FooTypeValue for scalar values so I am not sure the best way to connote a collection (maybe Column). I don't believe users will be impacted by this "API" change, since the class names were never explicitly part of the API.