Code style cleanup

[hadley]

I missed a few instances where we should use := in the tests. So probably worth generally doing another style pass once the the bulk of the PRs are merged.

Also think about new_external_generic()

Switch local_package() to take {} instead of ...:

downstream := local_package({
    own_generic := new_generic("x")
    Foo := new_external_class("upstream")
    method(own_generic, Foo) <- \(x) "from external class"
  })