Ephemeral RefCap shouldn't be an assignable refcap type

[redvers]

This allows modification of val and sending of mutable data to actors.

Example:

use "debug"
use "files"
use "collections"

actor Main
  var env: Env

  new create(env': Env) =>
    env = env'

    let map: Map[String, Array[String]] trn^ = recover trn Map[String, Array[String]] end
    map.insert("start", ["A" ; "b"])
    map.insert("A", ["end"])

    let romap: Map[String, Array[String]] val = map
    let foo: Foo = Foo(env, map)
    foo.test()

//    map.insert("X", ["c"; "b"; "end"; "start"])

actor Foo
  let env: Env
  let map: Map[String, Array[String]] val
  new create(env': Env, map': Map[String, Array[String]] val) =>
    env = env'
    map = map'

  be test() =>
    env.out.print("Size of map in actor: " + map.size().string())

[jasoncarr0]

A few extra cases for the fix:

• Unions: (Example iso^ | None)

• Intersections: A ref & A val already has a more aggressive check, but there could be other cases.
  (Side note: If this check is implemented, there's an argument that we should technically only enforce the ref & val check for variable bindings (as both A trn^ or A iso^ are already subtypes of A val & A ref by definition))

• Tuples (A iso^, B ref)

In other words, the test for capabilities has to be deep, not just checking a nominal cap

Some day also generics, but that has its own issue
#1931

[SeanTAllen]

I reopened this because the PR that closed it needs to be reverted for now.