Add NaiveDateTime::checked_(add|sub)_offset

[pitdicker]

These methods are const and public so they can be used with #1270.

Because the offset is never more than 24 hours, we can use NaiveDate::pred_opt() and NaiveDate::succ_opt() Instead of NaiveDate::add_days. This gives a nice little performance improvement for many methods on DateTime (benchmarks in #1069 (comment)).

Most of all we can use these methods to avoid out of range panics when converting datetimes from UTC to local or back.

Split out from #1069.

[codecov]

Codecov Report

Merging #1313 (4e441e6) into 0.4.x (a47e0e3) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            0.4.x    #1313      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.03%     
==========================================
  Files          38       38              
  Lines       17062    17120      +58     
==========================================
+ Hits        15568    15617      +49     
- Misses       1494     1503       +9     

Files Changed	Coverage Δ
src/offset/fixed.rs	73.13% <ø> (-7.58%)	⬇️
src/naive/datetime/mod.rs	97.35% <100.00%> (+0.05%)	⬆️
src/naive/datetime/tests.rs	98.77% <100.00%> (+0.13%)	⬆️

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[pitdicker]

Thank you, this made my day 😄.