Fuzzer: Avoid generating ref.as_non_null in more places

[kripken]

1. Rather than assert against this, allow it. We cannot prevent a situation
   where some nested struct.new in a global init ends up with a tricky
   field. Instead, we note the issue later, and recreate the global init.
2. That recreation logic had an ordering bug: First handle tuples, and
   then also handle ref.as_non_null, as we may end up creating one
   right there.

[tlively]

It might be nice to add a comment about why we can't assert we're in a funcContext here.

[kripken]

I think that is the general case, actually. We shouldn't be asserting on this. The other similar places need updating as well.