Closes #2617 -- overhaul dcast man page

[MichaelChirico]

#2617

Went ahead and axed all the NEW decorations around 4-year-old features:

2659496

25a74df

Generally just overhauled a lot of the wording which frames dcast.data.table as very closely related to reshape2 but I think it's very much its own thing by now... also reshape2 is dead:

reshape2 is retired

It may be worth deleting a benchmark vs. an obsolescent package as well? Updating with a comparison to tidyr?

[codecov]

Codecov Report

Merging #3542 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3542   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          66       66           
  Lines       12647    12647           
=======================================
  Hits        12290    12290           
  Misses        357      357

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[codecov]

Codecov Report

Merging #3542 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3542   +/-   ##
=======================================
  Coverage   97.17%   97.17%           
=======================================
  Files          66       66           
  Lines       12647    12647           
=======================================
  Hits        12290    12290           
  Misses        357      357

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7810b22...266e2c9. Read the comment docs.

[eddelbuettel]

@MichaelChirico @mattdowle Quick thanks for that -- I was reading ?dcast again today in the release version; glad to see this will get refreshed. Much appreciated.