arrow

ARROW-7035: [R] Default arguments are unclear in write_parquet docs

- Add documentation for `write_parquet` argument defaults
- Change default parquet compression from uncompressed (C++ default) to snappy (pyarrow's default)
- Add a test that this default is applied

I'm having trouble compiling the R package, so apologies if this doesn't pass tests.

Closes #5859 from karldw/master and squashes the following commits:

3c80054a9 <karldw> Export `ParquetFileWriter` and `ParquetWriterProperties`.
8bbdae3b6 <karldw> Clarify write_parquet and Codec docs, skip test if snappy unavailable.
6eb122354 <karldw> ARROW-7035:  Default arguments are unclear in write_parquet docs

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