Explicitly make `meta` a reserved name

[lognaturel]

It is my belief that it is generally impossible to introduce groups or fields with name meta but that this is a bit of an accident because of the way Validate works. We've used meta groups as part of the Entities spec and should explicitly reserve it.

For now we need to confirm the current state of things, figure out whether not it's actually helpful, and see whether it's likely safe.