Forbidding handles within the values of map<K, V> #751

I'd like to float the idea of adding further restrictions to the V in map<K, V> type, specifically forbidding any type that transitively includes a handle where a handle here is any of own<T>, borrow<T>, future<T>, stream<T>, error-context, etc. The original motivation for me bringing this up is bytecodealliance/wasmtime#14812, but the general problem is that map<K, V> is ABI-wise just a sequence of tuples, and there's nothing preventing overlap of the keys. Bindings generators thus need to handle the case of overlapping K values and properly destroy the provided V value. In the case of resource this might involve running destructors (own<T>), updating internal metadata in a store (borrow<T>), or possibly delivering async events (future<T> and stream<T>). These are all relatively weighty operations which are easy to forget (as Wasmtime has) and not always easy to integrate.

To address this I'd like to propose an interim workaround in the meantime of forbidding these types in the V of a map<K, V>. This would be a temporary restriction until someone has a compelling use case to support this and help fill out the implementation. This would be similar to how the K type is currently limited as well.