`length(::ImmutableDict)` is not `O(1)`

[adienes]

length(t::ImmutableDict) = count(Returns(true), t)

should ImmutableDict store its length as a field on construction? another resolution might be to delete the length method and add an IteratorSize() = SizeUnknown so that iteration doesn't try to call length.

or, just do nothing and accept a slow length call