feat: introduce 'weakmap' module in std

[jwerle]

weakmap::WeakMap is a higher level Map abstraction for borrowed references.

[aep]

Having two modules that do the exact same thing in upstream sounds confusing. I was assuming you just didnt like the map api and would like to change it, which is great.

[jwerle]

ah no this is just a small wrapper around map which is why I kept in userspace

[jwerle]

happy to close and keep there!

[aep]

allright. happy to accept api changes tho

[jwerle]

@aep cool set(), has(), and clear() are the only additions here, which just wrap map::{insert_bb,get,remove} methods

[aep]

yup, those are useful