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ARROW-7670: [Python][Dataset] More ergonomical API

- introduced `ds.field()`, `ds.scalar()`
- overloaded comparison and binary operators
- added `Expression.isin()` and `Expression.is_valid()`
- removed ScannerBuilder in favor of direct `Scanner` instantiation using keyword arguments
- added convenience methods for Dataset: `to_table()`, `to_batches()` and `scan()`
- added support for string scalar
- removed `InMemoryScanTask` and `DefaultPartitioning`

Closes #6268 from kszucs/filter-expr and squashes the following commits:

2bcf0554f <Krisztián Szűcs> fix review issues
706f3cc25 <Krisztián Szűcs> add to apidocs
a54a7c806 <Krisztián Szűcs> docstring
0966b8afa <Krisztián Szűcs> flake8
a950c975b <Krisztián Szűcs> flake8
9af49ce29 <Krisztián Szűcs> revert abbreviations
0170362cf <Krisztián Szűcs> cleanup
31cd5c60b <Krisztián Szűcs> remove ScannerBuilder
b0b4cd9b8 <Krisztián Szűcs> fix rename issue
b3438dfb2 <Krisztián Szűcs> cython specific overloading
dcab6b069 <Krisztián Szűcs> rename Expression to Expr
757343c13 <Krisztián Szűcs> string scalar
9c7ee5a03 <Krisztián Szűcs> improve the filter expression ergonomics

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>