Make permutations into forward range.

[jamesragray]

Since there was nothing preventing permutations from being a forward range, I have added a save method to make it into one. This is related to Issue 18975 - permutations.array creates an array of the original type without the permutations but doesn't fix it.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23362	normal	Permutations should be a forward range

Testing this PR locally

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

dub run digger -- build "master + phobos#8580"

[RazvanN7]

We should get this in as the spec specifically states that the return permutations returns a forward range. This should be filed as a bug.

[RazvanN7]

Why is this a draft?

[quickfur]

LGTM