std.format: Make formatting %f (and %g partly) 100% @nogc.

[berni44]

Followup on #7971 and #8000. See there for more details. See also #8005 and #8006.

[dlang-bot]

Thanks for your pull request and interest in making D better, @berni44! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21842	normal	std.format: "%-+05,g" adds extra comma

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#8007"