Closes #177

Unblocks #121

Batched select needs the source type to be broadcasted one level. It was actually impossible to specify this with the macro and axis based select previously used in device. This is because broadcasted select doesn't select from an axis at first, so you can't specify a single thing for it to select from.

Summary:

• Reworks the devices implementation to use a recursive trait implementation using recursive types to deconflict implementation.
  • These are controlled via unit structs called "Modes"
• Adds (private) generic select function that uses select modes to control behavior
• Rework existing tensor impls to use modes
• Adds a SelectBatchAx0 trait with select_batch function for 1d/2d/3d tensors