Expose Universal WeakMap

[dy]

Hi!
UniversalWeakMap can be useful on its own, a good replacement for primitive-pool.
Can that possibly be exported as a separate entry?

import { UniversalWeakMap } from 'immutable-tuple'

[dy]

@benjamn any thoughts?

[fregante]

Ideally it should be its own module. I've been wanting to publish my own and I was surprise to see it in your code.