Faster map w/ input_columns & faster slicing w/ Iterable keys
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@lhoestq Just fixed the code style issues— I think it should be good to merge now :)
@lhoestq Fixes #2193 

- `map` now uses `with_format` to only load needed columns in memory when `input_columns` is set
- Slicing datasets with Iterables of indices now uses a new `Table.fast_gather` method, implemented with `np.searchsorted`, to find the appropriate batch indices all at once. `pa.concat_tables` is no longer used for this; we just call `pa.Table.from_batches` with a list of all the batch slices.

Together these changes have sped up batched `map()` calls over subsets of columns quite considerably in my initial testing.
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