new update doesn't really do the right thing when updating a term to a constructor

[mitchellwrosen]

If you add

foo.bar : Nat
foo.bar = 17

and then put

unique type foo = bar

into scratch.u, you'll see a scary message that says update will fail:

  Loading changes detected in ~/unison/trunk/scratch.u.


  I found and typechecked these definitions in ~/unison/trunk/scratch.u. If you do an `add` or
  `update`, here's how your codebase would change:

    x These definitions would fail on `add` or `update`:

      Reason
      blocked type foo
      ctor/term collision   foo.bar

      Tip: Use `help filestatus` to learn more.

... but update doesn't fail.

> update

  Okay, I'm searching the branch for code that needs to be updated...


  Done.

Now we have one name foo.bar that refers to two things.

> view foo.bar

  type foo = bar#hj9bfbk48b#0

  foo.bar#cq22mm4sca : Nat
  foo.bar#cq22mm4sca = 17