Feature/update date code

[eddelbuettel]

src/Date.cpp basically dates from 2010.

It contains some stub code we don't use (ie to load a zonefile) but maybe could / should, but that is for another day.

This PR mostly brings in an update to get to what R uses there. Ultimately I should farm this out but one attempt in repo rapidatetime got suck. Maybe I'll get back to this.

The PR should be harmless. It tests out fine, and the branch also supports tests of RQuantLib which is one the few packages actually using Rcpp::Date a little...

[codecov-io]

Current coverage is 76.35% (diff: 64.07%)

Merging #621 into master will decrease coverage by 0.12%

@@             master       #621   diff @@
==========================================
  Files            68         67     -1   
  Lines          4004       4017    +13   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           3062       3067     +5   
- Misses          942        950     +8   
  Partials          0          0          

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[eddelbuettel]

@kevinushey @thirdwing @jjallaire @nathan-russell -- Happy New Year!

In case any one of you has a minute or two to spare, a quick code review here would be appreciated.
Should not bite in any way. New unit test tests in the this branch and in main.

[eddelbuettel]

And I just added two more.

[kevinushey]

LGTM!