Implement sum(f, ::AbstractFill)

[devmotion]

I noticed that sum(f, ::AbstractFill) is currently not implemented and added it in this PR, together with some tests and a version bump.

[codecov]

Codecov Report

Merging #132 (a2ae76c) into master (ba49d16) will decrease coverage by 0.15%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
- Coverage   91.36%   91.20%   -0.16%     
==========================================
  Files           4        4              
  Lines         463      466       +3     
==========================================
+ Hits          423      425       +2     
- Misses         40       41       +1     

Impacted Files	Coverage Δ
src/FillArrays.jl	91.63% <66.66%> (-0.29%)	⬇️

---

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 ba49d16...a2ae76c. Read the comment docs.