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ARROW-8001: [R][Dataset] Bindings for dataset writing

* r/R/dataset.R is broken out into smaller files. I did this in the first commit, isolated from the behavior changes, so if you do a diff without the first commit, it's easier to see what has changed
* Normalize paths in `write_dataset()` as was done in `open_dataset()` in ARROW-9743
* Add bindings to create `InMemoryDataset` and use those in `write_dataset()` to enable you to write a `data.frame`, `RecordBatch`, or `Table`
* Allow writing a subset of columns, and gather information from a previous `select()` call to do that by default. Renaming columns is not supported

Closes #8041 from nealrichardson/r-dataset-writing2

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