feat: effects can spawn new runs, key page and block reads

[harrysolovay]

Let's say you have the following effect:

 const decoded = sys.atom(
  "Anonymous",
  [$key, keysEncoded],
  (keyCodec, keysEncoded) => {
    return keysEncoded.map((keyEncoded) => {
      return keyCodec.decode(U.hex.decode(keyEncoded));
    });
  },
);

You can refactor the impl to a function function...

 const decoded = sys.atom(
  "Anonymous",
  [$key, keysEncoded],
- (keyCodec, keysEncoded) => {
+ function(keyCodec, keysEncoded) {
    return keysEncoded.map((keyEncoded) => {
      return keyCodec.decode(U.hex.decode(keyEncoded));
    });
  },
);

... which gives access to the executor via this:

const decoded = sys.atom(
  "Anonymous",
  [$key, keysEncoded],
  function(keyCodec, keysEncoded) {
+   this.run(anotherEffect); // would likely––in practice––await and use the result
    return keysEncoded.map((keyEncoded) => {
      return keyCodec.decode(U.hex.decode(keyEncoded));
    });
  },
);

Additionally, this PR cleans up the key page and block read effects with appropriate decoding.