Custom key validation in records

[mary-ext]

This is easily implemented by chaining my own assertions on top, but I think it's still worth suggesting:

It would be nice if I could pass a custom key type to records, e.g. here in this case I want to limit the length of keys used in a record.

const document = v.object({
  services: v.record(
    v.object({
      type: v.string(),
      // ...
    }),
    v.string()
      .assert((input) => input.length <= 32, `service id too long (max 32)`),
  ),
});