[R] Implement halffloat -> R conversion

[asfimport]

Followup to ARROW-3783

Reporter: Neal Richardson / @nealrichardson

Related issues:

• [C++] Cast to/from halffloat not implemented (is blocked by)

_{Note: This issue was originally created as ARROW-7753. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
@nealrichardson is this important soon or just "nice to have"?

[asfimport]

Neal Richardson / @nealrichardson:
AFAIK it's not urgent. Original issue is 1.5 years old. I just picked it up because I thought it would be a quick win; didn't realize that halffloat was so sparsely implemented in Arrow C++.

[vkhodygo]

Is it still actual? I noticed I can't read halffloat values correctly when I load a parquet file in R. Not sure if I need to open a new issue.

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.

[vkhodygo]

@nealrichardson Is it completely abandoned?

[nealrichardson]

Looking at the linked #20213, the C++ library appears to have some float16 support now, so this should be doable. I don't personally intend to pick this up but someone could.