Move datasets to MLDatasets.jl

[darsnack]

In the long term, we'd like most of the src/datasets code to move to MLDatasets.jl. To make this happen, we need a refactor of MLDatasets.jl to be more extensible and build on top of LearnBase.jl. Below is the structure envisioned for MLDatasets.jl:

1. Low-level API: structs for different types of I/O (e.g. FileDataset) that support reading from the underlying I/O via getobs and nobs from LearnBase.jl
2. High-level API: specific datasets (e.g. CIFAR10) implement using the low-level API

To achieve this goal, we need to complete the following stages:

• Move data containers (e.g. FileDataset) to MLDatasets.jl
• Move data container transformations (e.g. mapobs, groupsobs, etc.) to MLDataPattern.jl (these transformations apply generically to any iterator of observations, not just data containers)
• Refactor existing data sets in MLDatasets.jl to utilize the low-level APIs
• Move FastAI.jl datasets to MLDatasets.jl

[lorenzoh]

Closed by #229