remove send/sync retrictions when possible

[TyOverby]

Because HamtMap isn't Send/Sync, there's no reason to make K or V Send or Sync.

This removes the necessity in the API, while allowing for the possibility of a Sync/Send HamptMap in the future.

[Eh2406]

I am still getting

     = help: the trait `std::marker::Sync` is not implemented for `std::rc::Rc<...>`
     = note: required by `hamt_rs::ShareStore`

with hamt-rs = { git = "https://github.com/TyOverby/hamt-rs", branch = "remove-reqs"}. Am I doing something wrong?