Duration features part 1

[pitdicker]

Split our from #1137.

• Derive Default: Consider adding a "Default" implementation to Duration #717
• Add a subsec_nanos method: Provide access to subsecond nanos in the public interface of Duration. #154
• Add try_* builders: Fix panic in DateTime::checked_add_days #941
• Implement AddAssign and SubAssign: [Feature Request] AddAssign/SubAssign implementation to chrono::Duration #1054

[codecov]

Codecov Report

Merging #1327 (14d4669) into 0.4.x (604b92a) will increase coverage by 0.02%.
The diff coverage is 97.61%.

@@            Coverage Diff             @@
##            0.4.x    #1327      +/-   ##
==========================================
+ Coverage   91.50%   91.53%   +0.02%     
==========================================
  Files          38       38              
  Lines       17314    17325      +11     
==========================================
+ Hits        15844    15859      +15     
+ Misses       1470     1466       -4     

Files	Coverage Δ
src/naive/time/mod.rs	96.53% <100.00%> (ø)
src/duration.rs	92.88% <97.50%> (+1.02%)	⬆️

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