Use KeySet instead of KeyIterator

[martinholters]

...in argument types of in and next on newer Julia versions.

Admittedly, I don't know whether the KeySet interface promises anything that DataStructures currently doesn't hold, but at least this makes DataStructures load again on Julia 0.7.

[codecov]

Codecov Report

Merging #349 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #349   +/-   ##
=======================================
  Coverage   96.45%   96.45%           
=======================================
  Files          31       31           
  Lines        2286     2286           
=======================================
  Hits         2205     2205           
  Misses         81       81

Impacted Files	Coverage Δ
src/default_dict.jl	97.22% <100%> (ø)	⬆️
src/ordered_dict.jl	86.8% <100%> (ø)	⬆️

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[oxinabox]

LGTM,
what about SortedDict ?

[StephenVavasis]

I haven't understood what this thread is about. What is the issue or problem for SortedDict?

[oxinabox]

It is not a problem for SortedDict (but I thought it might be).
Base has changed the name of KeyIterator to KeySet.
So if keys(::StortedDict) was returning a KeyIterator then it would need to be updated

But actually I see that keys(::SortedDict) returns its own type:
DataStructures.SDMKeyIteration
so I guess it does not need to be changed.

So I think this can be merged now

[martinholters]

Worth tagging a new release to have a version working on current Julia master?