off-by-one namespace layout

[aryairani]

Currently we have:

data Namespace = 
  Namespace {
    terms :: Relation NameSegment TermReference,
    types :: Relation NameSegment TypeReference,
    children :: Map NameSegment NamespaceHash
  }

This avoids unnamed definitions at the root, because all definitions need at least one NameSegment.

but we could have had

data Namespace =
  Namespace {
    terms :: Set TermReference,
    types :: Set TypeReference,
    children :: Map NameSegment NamespaceHash
  }

The latter simplifies some operations, such as finding, moving, or deleting all the definitions with a certain prefix.

We can link related issues to this ticket.

[aryairani]

Maybe could get rid of the Split data structures outside command parsing/validation maybe, for making sure we don't get unnamed definitions at the top level