feat: event extraction util

[harrysolovay]

Addresses functionality described in #272 (not necessarily the API design though).

const events = C
  .events(extrinsic, hash)
  .bind(env)();

One noteworthy change:

No longer decoding block extrinsics by default –– this behavior is opt / one must pass the hexes to the extrinsicsDecoded effect. Aka.

const extrinsicsRaw = C.chain.getBlock(C.polkadot)()
  .access("block")
  .access("extrinsics");
const root = C.extrinsicsDecoded(T.polkadot, extrinsicsRaw);

We'll simplify this once we can attach methods to zones. Ie., this is our north star:

- const extrinsicsRaw = C.chain.getBlock(C.polkadot)()
-   .access("block")
-   .access("extrinsics");
- const root = C.extrinsicsDecoded(T.polkadot, extrinsicsRaw);
+ const root = C.chain.getBlock(C.polkadot)().extrinsics