Misc. `namespace.interleave` issues

[distractedlambda]

I've started trying to use namespace.interleave, and have hit a few minor bugs so far:

1. The ~name option is not documented beyond its existence, and also does not seem to work (the implementation stumbles over parsing it).
2. The block after the ~init option is documented as being optional, but non-block usage does not seem to be parsed correctly.
3. namespace_meta.Exports.Declaration is documented as taking an Identifier, but it does not seem to apply the usual syntax-object coercions (I have to call Syntax.ephemeral_term first in my usage). Also, it seems like that should be generalized to allowing any Name.

Also, would it be possible to make the body a required option as well (so that the only groups immediately nested under namespace.interleave are keyword-prefixed options?). Otherwise, a macro that expands to namespace.interleave needs to check that that body doesn't start with anything that would be recognized as an option (which would usually be erroneous anyways, but still).

[mflatt]

I added ~body as an optional form, but it's not required, and it prefixes any body forms that are supplied after options.