ARROW-15800 [R] bindings for lubridate::as_date() and lubridate::as_datetime() - abandoned

[dragosmg]

Update: I have abandoned this PR. The work continues at #12738 and focuses solely on adding bindings for as_date() and as_datetime().

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What is this PR trying to do:

• add bindings for lubridate::as_date() and lubridate::as_datetime()
• reorganise the datetime bindings by:
  • creating register_bindings_datetime_helpers() which brings together the datetime coercion functions
  • move difftime functions to register_bindings_duration()

[github-actions]

https://issues.apache.org/jira/browse/ARROW-15800

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[jonkeane]

This isn't really possible without a lot of extra work, but for the future: when moving lots of stuff around like this, it can be helpful to do the reorganization as either the first or the last commit, so that the real diff can be seen more easily.

For the stuff that is being moved around, you did just that, move them around without changing the functionality, yeah? The only new functionality is as_date, as_datetime and the corresponding tests, yeah?