Example of hash map with GC cell as key and/or value

[ptomato]

Conversation starting here: https://chat.mozilla.org/#/room/#spidermonkey:mozilla.org/$YXBhZxFLHDHPc9pRp6Jz2-Vm9EDbzec0Rbeig_fPtJM

Especially:

sfink: Yes, a [GCHashMap<JS::Heap<JSString*>>] seems wrong to me if any of the string keys could be nursery strings.
DefaultHasher<JSString*> just uses the pointer
one option would be to hash and match on the string contents. We have a couple of instances of that in the tree. One for string deduplication, one for counting duplicate strings for about:memory.
tcampbell: what version did we turn on nursery strings?
this probably needs a good note and example in the examples repo..
sfink: [...] I was surprised that we make so little of strings as keys; there's not much prior art in the tree.
it looks nursery strings were Fx67, from bug 1442481

We should have an example of these sorts of hash maps using MovableCellHasher.