ARROW-5600: [R] R package namespace cleanup

[nealrichardson]

This is instead of #4491, without the function naming change that we wanted to think about more intentionally.

It also removes a few lingering references to tibble in the package, which were still passing in tests because tibble is in Suggests and the test hosts install all of the Suggests packages.

@romainfrancois

[codecov-io]

Codecov Report

Merging #4566 into master will decrease coverage by 12.83%.
The diff coverage is 75%.

@@ Coverage Diff @@## master #4566 +/- ##
===========================================
- Coverage 88.15% 75.31% -12.84% 
===========================================
Files 855 56 -799 Lines 107013 3192 -103821 Branches 1253 0 -1253 ===========================================
- Hits 94333 2404 -91929 + Misses 12416 788 -11628 + Partials 264 0 -264

Impacted Files	Coverage Δ
r/R/arrow-package.R	100% <ø> (ø)	⬆️
r/R/R6.R	75% <0%> (ø)	⬆️
r/R/RecordBatch.R	96% <100%> (ø)	⬆️
r/R/Table.R	86.66% <100%> (ø)	⬆️
r/R/read_table.R	100% <100%> (ø)	⬆️
r/R/array.R	72% <0%> (-4%)	⬇️
python/pyarrow/ipc.pxi
cpp/src/arrow/csv/chunker-test.cc
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
... and 795 more

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[romainfrancois]

Thanks. I'll finish this off and merge soon.