feat: rpc subscription refactor + returning results

[harrysolovay]

This PR refactors RPC subscriptions / the Capi-wide listener interface such that one can return (and infer types from) results.

Previously...

const root = C.chain.unsubscribeNewHeads(T.polkadot)(
  C.chain.subscribeNewHeads(T.polkadot)([], function(header) {
    if (someCondition)
      return this.stop()
  }),
)

With these changes...

const root = C.chain.subscribeNewHeads(T.polkadot)([], (ctx) =>
  (header) => {
    if (someCondition)
      return ctx.stop()
  })

The calling of a corresponding unsubscribe call method is now taken care of at the RPC level. This means we no longer need to return the subscription ID from RPC subscription effects, which allows us to return other values.

const header = C.chain.subscribeNewHeads(T.polkadot)([], (ctx) =>
  (header) => {
    if (header.number > THRESHOLD)
      return ctx.stop(header)
  })

header can be utilized as any other effect. This enables myriad new possibilities for modeling effects that observe and react to on-chain events.

[kratico]

LGTM