release datafrog 2.0.0

[nikomatsakis]

• Breaking changes:
  • leapjoin now takes a tuple of leapers, and not a &mut slice:
    • from_leapjoin(&input, &mut [&mut foo.extend_with(...), ..], ..) becomes
      from_leapjoin(&input, (foo.extend_with(...), ..), ..)
    • if there is only one leaper, no tuple is needed
  • Relation::from now requires a vector, not an iterator; use
    Relation::from_iter instead
• Changed the API to permit using Relation and Variable more interchangeably,
  and added a number of operations to construct relations directly, like Relation::from_join
• Extended leapfrog triejoin with new operations (PrefixFilter and ValueFilter)