Don't allow interfaces to have private methods

[SeanTAllen]

When Pony allowed interfaces to have private methods, it was possible to use an interface to break encapsulation for objects and access private methods from outside their enclosing package.

The following code used to be legal Pony code but will now fail with a compiler error:

actor Main
  new create(env: Env) =>
    let x: String ref = "sailor".string()
    let y: Foo = x
    y._set(0, 'f')
    env.out.print("Hello, " + x)

interface Foo
  fun ref _set(i: USize, value: U8): U8

AsioEventNotify in the standard library had to be changed from an interface with a single private method to a trait in order to conform with this change.

Fixes #2772

[SeanTAllen]

I expect this to pass CI. If it does it needs:

• release notes
• comment for this PR clean up
• Good PR comment
• bad pony test to verify the error
• Changelog label
• Additional manually added changelog entry (for ASIO change)