feat!: require a seal to return synchronously

[jo16oh]

Currently, an async seal is accepted, and it makes every derivation async:

const Reading = Val.companion<Reading>()
  .implSeal(async (r, seal) => seal(await normalize(r)))
  .impl();
Reading.seal({ n: 1 }); // Promise<Reading>
Reading.patch(reading, { n: 2 }); // Promise<Reading>

• implSeal rejects a seal whose return type includes a PromiseLike, on a Val companion, an Enum companion and a variant: "a seal is synchronous; await before sealing"
• A seal whose return type is any is still accepted
• Breaking: an async seal is now rejected. Do the async work first and pass its result to the seal