Supporting Permission Operations like Intersection `&`

[theronic]

EACL currently only supports the + Union operator, e.g.

definition user {}

definition server {
  relation owner: user
  relation viewer: user

  permission view = owner + viewer
  permission delete = owner
}

But EACL does not Negation or Intersection yet. You can usually work around this with application logic by doing two permissions checks and doing the AND / OR logic in your code, but it's not ideal for consumers who need those Spice operations.

Thinking about how to support this efficiently. With better schema AST, should be possible, but may complicate the implementation quite a bit.