...and don't let them leak out of their evaluation contexts.
Also keep the exceptions separate from the actual values.
This greatly reduces the number of assertions hit while entering random
data into a sheet.
This is pretty heavy and unoptimized, but it will do the trick for now.
Basically, Heap now has a HashTable<HandleImpl*> and you can call
JS::make_handle(T*) to construct a Handle<T> that guarantees that the
pointee will always survive GC until the Handle<T> is destroyed.
