Document panics in Add/Sub impls and use expect

[pitdicker]

A large but simple PR.

I made sure all Add/Sub documentation on NaiveDate, NaiveTime, NaiveDateTime and DateTime follows the same format and mentions panics.

The implementations now use expect instead of unwrap.
And I changed impl Add<std::time::Duration> for NaiveTime and Sub to take a modulus of the duration before converting to remove a panic case.

Fixes #1302, #1314.

[codecov]

Codecov Report

Merging #1316 (cdecf2a) into 0.4.x (d450b68) will increase coverage by 0.02%.
The diff coverage is 92.10%.

@@            Coverage Diff             @@
##            0.4.x    #1316      +/-   ##
==========================================
+ Coverage   91.25%   91.27%   +0.02%     
==========================================
  Files          38       38              
  Lines       17114    17121       +7     
==========================================
+ Hits        15617    15627      +10     
+ Misses       1497     1494       -3     

Files Changed	Coverage Δ
src/datetime/mod.rs	84.97% <50.00%> (-0.26%)	⬇️
src/naive/datetime/mod.rs	97.35% <93.93%> (+<0.01%)	⬆️
src/naive/date.rs	96.20% <100.00%> (+<0.01%)	⬆️
src/naive/time/mod.rs	96.53% <100.00%> (+0.60%)	⬆️

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[djc]

FWIW all these impl docs are nice to have, I guess, but I would consider them very low priority.

[pitdicker]

I would consider them very low priority.

Agreed 😄