More progress on capture-checking stdlib - #26727

We had to make Source and Codec function variables non-capturing, but they seem like good choices anyway.
Sources cannot be created from an impure Iterable, but doing that seems wrong anyways as `withReset` would re-create the iterator.

Would be interesting to try making Console/StdIn a global capability.

In particular, we require numeric and comparison traits (Ordering, Ordered, Numeric, Equiv) to be pure.

Of note: `typeConstraints` needs some change.

ReferenceWrapper marked Pure, as it is supposed to wrap a generic reference.