fix accumda() so it does less recursion and more iteration

[WalterBright]

Based on @ibuclaw 's observation https://issues.dlang.org/show_bug.cgi?id=6401#c4 that accumda was doing too much recursion.

[dlang-bot]

Thanks for your pull request, @WalterBright!

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Testing this PR locally

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dub run digger -- build "master + dmd#12455"

[ibuclaw]

Are benchmarks being done between each change? Can results be posted with each of these commits so we know what kind of effect it has?

[WalterBright]

This one just does some tail recursion instead of recursing. It uses less stack, which is faster. It's a commonplace optimization.