[R] Refactor collect and compute methods

[asfimport]

ARROW-11754 added a compute() method which calls collect(as_data_frame = FALSE). This was a clean and simple way to add compute(), but it would be more logical if collect() instead called compute(), or if both called an internal function that forced computation of the lazy ops. But this is complicated by the need to call restore_dplyr_features() in both compute() and collect() and muddied by some uncertainty about where exactly ensure_group_vars() and ensure_arrange_vars() should be called. Take a closer look at this later date and consider whether to refactor it.

Reporter: Ian Cook / @ianmcook
Assignee: Neal Richardson / @nealrichardson

Related issues:

• [R] Add as.data.frame.Dataset method (is fixed by)

_{Note: This issue was originally created as ARROW-12282. Please see the migration documentation for further details.}

[asfimport]

Neal Richardson / @nealrichardson:
Doing in ARROW-15460.