Use QuantEcon.next_k_array! in support_enumeration

[monsij]

Closes issue #70
The remaining changes have been made mentioned in the comment-link. Kindly let me know if the changes in docstrings are also required.
@sglyon @oyamad

[oyamad]

@monsij Thanks.

• Yes, please delete the docstrings of _next_k_combination and _next_k_array!.
• Please do not delete blank lines in other docstrings. See the changes you made.

[codecov-io]

Codecov Report

Merging #82 into master will increase coverage by 3.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   92.52%   96.43%   +3.91%     
==========================================
  Files           6        6              
  Lines         468      449      -19     
==========================================
  Hits          433      433              
+ Misses         35       16      -19

Impacted Files	Coverage Δ
src/support_enumeration.jl	100% <ø> (+21.83%)	⬆️

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[coveralls]

Coverage increased (+3.9%) to 96.437% when pulling 5453b2b on monsij:support_enumeration into b921342 on QuantEcon:master.

[monsij]

It's done ! 🎉 @oyamad @sglyon

[oyamad]

Great, thanks!

[monsij]

My pleasure!