Add `UseResult` annotation to `rebuild` (#631)

This catches issues like #630 where we forgot to use return value of
`rebuild`. Since `rebuild` does not update in-place, not using the
return value is often a bug.

This adds `meta` as a dependency but it should not affect generated
code. We may also want to use `meta` in the future for things like:

- Annotating `GeneratedMessage` methods that are supposed to be used by
  the generated code with `@protected`.

- Annotating generated message classes with `@sealed`. This help with
  maintaining backwards compatibility when adding new members to
  generated messages or to `GeneratedMessage`.