Support auto-completion for enums

[jneem]

nls should support auto-completion for enum tags, especially when we know the expected set of allowed tags, and possibly also falling back to all the "known" enum tags (whatever that means exactly).

For example, in

let C = { type | [| 'foo, 'bar, 'baz |] } in
{
   type = 'fo
} | C

we should auto-complete 'foo.