C# ResourceTable never frees entries #2056

ResourceTable<T> in Runtime.cs adds an entry for every Give and Lend. Take and the end of a call set the entry's value to null but never remove the entry. A long-lived sandbox that passes resources across calls therefore grows by one entry per transfer, without limit.

Storing entries in a dictionary keyed by an increasing counter, and removing them on Take and at the end of a call, bounds the table to live entries. Handles are still never reused.