Add bit-slice to enum for issue 992

[MollyDream]

• Add the bit-slice operator to enum.
• Clarify when bit-slice can produce an l-value. Fixes #992

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[jnfoster]

In the interest of tidying up the set of active issues on the P4 specification repository, I'm marking this as "stalled" and closing it. Of course, we can always re-open it in the future if there is interest in resurrecting it.