arrow

ARROW-7963: [C++][Dataset][Python] Expose Dataset Fragments to Python

In C++ adds `Scanner::GetFragments`, currently only used from python.

In Python adds class `Fragment` and subclass `FileFragment`. The former has only a `partition_expression` property, the latter adds a `path` property.

Also in C++: deletes unused FileFragment subclasses.

Closes #6570 from bkietz/7963-Expose-listing-fragments and squashes the following commits:

e0fb05c14 <Benjamin Kietzman> add root_partition to expected fragment partitions
f915d7bf9 <Benjamin Kietzman> add test for fragment partition expressions
275fbb3a4 <Benjamin Kietzman> fix merge error: spurious assertion
9eae9f33f <Benjamin Kietzman> GetFragments -> get_fragments
bdb1b9e70 <Benjamin Kietzman> Update python/pyarrow/_dataset.pyx
b8c2ba6eb <Benjamin Kietzman> Update python/pyarrow/_dataset.pyx
68517d8ce <Benjamin Kietzman> ARROW-7963:  Expose Dataset Fragments to Python

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>