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ARROW-3591: [R] Support for collecting decimal types

Fix for https://issues.apache.org/jira/browse/ARROW-3591, implemented following Python string to decimal conversion: https://github.com/apache/arrow/blob/master/cpp/src/arrow/python/arrow_to_pandas.cc#L723.

Note: We could consider a more efficient conversion from decimal to R's integer or numeric types depending on the precision/scale from Arrows decimal; however, my guess is that this has been explored before for other languages and there is no straightforward `memcpy()` version of cross language decimal type conversions since the internal representation has been implemented as language specific.

Bonus: Added accessors in the `schema` wrapper for `num_fields()` and `field()` which can help diagnose schema issues.

Author: Javier Luraschi <javierluraschi@hotmail.com>

Closes #2819 from javierluraschi/feature/collect-decimal and squashes the following commits:

efabe274 <Javier Luraschi> fix lint style suggestions
d6ea85bf <Javier Luraschi> code review feedback
5487cb3b <Javier Luraschi> rebuild rcpp bindings
437fed57 <Javier Luraschi> support collecting decimal types
f2412f6d <Javier Luraschi> add num_fields and field methods to schema r6 class