Add inline query and mutation blocks

[sujayakar]

await step.inlineQuery(async (ctx) => {
  ...
});
let id;
await step.inlineMutation(async (ctx) => {
  const doc = await ctx.db.get(id); // can reference things in closure
  id = doc.nextId; // can even set things in closure w/o affecting determinism.
  await ctx.db.patch(doc._id, { nextId: null });
});

to maintain determinism, we'd journal each operation in a block.

this may interact poorly with transaction size limits -- we may want to find a way to be defensive and throw an error before the workflow mutation hits the global limit.