Infer datashape of slice of dataset

[mrocklin]

I want something like the following. Do we have this already somewhere?

>>> ds = dshape('var * {name: string, amount: int}')
>>> index = (slice(0, 5), 'amount')

>>> dshape_of_subset(index, ds)
'5 * int'

>>> dshape_of_subset(0, ds)
'{name: string, amount: int}'

CC @mwiebe