WeakKeyIdDicts

Implements a WeakKeyIdDict which constructs a hash table where the keys are weak references to objects that may be garbage collected even when referenced in a hash table.

It defines one type, WeakKeyIdDict, that follows the same API as Dict.

_tmp_key = [1]
wkd = WeakKeyIdDict(_tmp_key => 1)
let tmp = [42]
    wkd[tmp] = 2
    @show length(wkd) # 2
end
# at this point there is no strong reference left to the vector [42]
# previously reachable via tmp
GC.gc(true)

@show length(wkd) # 1