Turns out we've never supported them, neither directly constrained nor
with constrained members, because of a lack of tests. Yet another data
point to prioritize working on code-generating `constraints.smithy` (see
#2101).

The implementation is simple: we just need to call the symbol provider
on the member symbols instead of on the target symbols so we get
`Option<T>` list members / map values if applicable, and handle the
wrapper when converting between unconstrained and constrained types with
help from `match` and `Option<T>::map`.

…lists

…lists