Remove reshape2 suggest

[mattdowle]

Closes #3080
Completes final item in #2740

Added comments to the top of fmelt.c as to why we don't import reshape2 (R dependency inheritance)

melt is generic in reshape2 so I'm not sure why we were explicitly calling it

dcast dispatches non-data.table to non-generic reshape2::dcast the same as before, but with a more helpful message if reshape2 is not installed.

Tidied NAMESPACE. There were a few methods explicitly exported which didn't need to be since S3method already exports the method. This leaves just one use of the new delayed S3method registration, for xts.

The as.Date method has another twist in that zoo::as.Date masks base::as.Date and they are both generic.

[codecov]

Codecov Report

Merging #3081 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3081      +/-   ##
==========================================
+ Coverage   90.86%   90.88%   +0.02%     
==========================================
  Files          61       61              
  Lines       11808    11804       -4     
==========================================
- Hits        10729    10728       -1     
+ Misses       1079     1076       -3

Impacted Files	Coverage Δ
R/fcast.R	83.79% <ø> (+0.92%)	⬆️
R/fmelt.R	95% <100%> (+2.14%)	⬆️

---

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 125416c...a32c5e5. Read the comment docs.

[jangorecki]

is this really required? this was exported in 1.11.6 #2920 as per user request in a comment somewhere. As it is very recent it won't break much code, but NEWS entry would be useful.

[mattdowle]

A comment next to it explaining why and when exported would have been good because it's not proper to do so in addition to the S3method(split, data.table) which is still there. Should we consider exporting all methods for base generics as well as registering them?

[mattdowle]

Added a news item to 1.11.8 : #3083