7689 - permit metadata on Mu literal integers

Metadata is always ignored. It's purely for documentation purposes. But
as long as Mu has no named constants it's starting to feel increasingly
essential.

I'm still not going to bother to add metadata to other parts of the language.
Let's see if we need them. Even though it's a little warty that the rules
vary throughout the stack:

  - bare SubX: metadata everywhere
  - SubX with syntax sugar: no metadata in calls or addressing-mode sigil-expressions
  - Mu: metadata only for literal integers

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