Add feature gates for `for` and `?` in consts
These operations seems *relatively* straightforward to support, and only seem to be blocked on `impl const Trait`.
I have included a working test for `const_try`, but `const_for` is currently unusable without reimplementing *every single* defaulted `Iterator` method, so I didn't do that.
(both features still need tracking issues before this is merged)
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