Make some private data private
Make some private data private

Partially addresses #7 by making `queue` and `handlers` use `Symbol`s as
property keys. The data can still be accessed with `Reflect.ownKeys()`,
but using `WeakMap` to manage private data slowed things down dramatically.

lance/fidelity,bucharest-gold/fidelity,bucharest-gold/fidelity,lance/fidelity,bucharest-gold/fidelity