Refactor std.algorithm.iteration

[andralex]

The refactorings involve simplifying code that was working around compiler bugs, pushing symbols from top level to where they are needed, and a few more simplifications.

[dlang-bot]

Thanks for your pull request, @andralex!

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Testing this PR locally

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dub run digger -- build "master + phobos#7638"

[dukc]

Good work - just a little nicer to read all over the module. Modulo getting the test suite to pass, LGTM.

[andralex]

whoa blast from the past

[RazvanN7]

@andralex what are your plans with regards to this PR?