Add merge and friends with arbitrary number of args

[pablosanjose]

Closes #129

adds methods for merge, merge!, mergewith and mergewith! with arbitrary number of AbstractDictionary args.

[andyferris]

Excellent - this is a great idea. Thanks for the contribution.

I think we should address Julia 1.0 compatibility (either drop it, or maintain it) and remove the Callable constraint, but other than that it looks good to me.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7b5b7b3) 79.35% compared to head (81a3062) 80.10%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   79.35%   80.10%   +0.74%     
==========================================
  Files          21       20       -1     
  Lines        2349     2357       +8     
==========================================
+ Hits         1864     1888      +24     
+ Misses        485      469      -16     

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[pablosanjose]

Thanks for the quick reply and review!
Regarding upping the compatibility requirement to 1.6, it's your call. I would personally do it, since 1.6 has been out very long now. Please let me know.
EDIT: Thinking about that again, I believe it's better practice to do the compat change in a separate PR.

[andyferris]

Re Julia 1.6, the question I have is this a breaking change? Do we need a major version bump? I suppose the package manager will deal with it for us.

[pablosanjose]

Nope, it's not a breaking change (with the last commit). It can be argued that it is a bugfix (so bugfix release), since it just makes merge and co on AbstractDictionaries compliant with the API defined by Julia for AbstractDict.

[andyferris]

Sorry, my comment was about changing the minimum compatible Julia version to 1.6. In any case, such a decision can be dealt with seperately. (I agree this is more of a bugfix).

Thanks again!