PEP 814: Add frozendict built-in type

[Nineteendo (Nice Zombies)]

@blhsing

I think it makes sense to optimize for the use case by adding a dict.detach method, which creates a frozendict that reuses the hashtable of the dict and make the original dict point to a new empty hashtable

If we execute fd = frozendict(d), can’t we make make d point to the hash table of fd and only copy it when we add, change or remove a key (but not when we clear the dict). That would give us O(1) construction without changing the class and would also work for d = dict(fd).