Fix Issue 6447: iota(BigInt) too

[Biotronic]

This adds the missing overload for three-parameter iota as pointed out in Issue 6447, and should be enough to finally close the issue.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	6447	enhancement	iota(BigInt) too

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + phobos#5745"

[Biotronic]

As @wilzbach suggested, I united all iota overloads. This has the benefit of uniting functionality - where previously different overloads of iota supported different operations, now these operations are available on any iota overload whose parameter types support the necessary operations.

[DmitryOlshansky]

Autotester is not happy(

[Biotronic]

Believe me, I know. I've failed to get some of the unittests to run on my local machine, and the specific one that fails now works when I copy the code to a separate file.

[MetaLang]

I'd suggest we leave uniting the overloads to a separate PR and get the fix merged.

[Biotronic]

After repeated failed attempts at getting the unit tests to run, I decided to revert to the simple change that fixes 6447, and a greater rewrite will have to wait.

[JackStouffer]

Ping @andralex

[quickfur]

ping @andralex Do the changes look OK now? Apparently this has stalled. Let's get it moving again!

[thewilsonator]

Ping @andralex this needs a rebase but is this otherwise goo to go?