Use the main name for trait interface protocols/interfaces

[bendk]

When you have a trait interface named Foo without with_foreign we currently name things like this:

• FooProtocol -- the protocol for the trait (there's a couple variations on this, like FooInterface)
• Foo -- the name of the type that implements the trait by calling into Rust.

I think we should swatch to using Foo and FooImpl for these items, i.e. swapping which thing gets the main name. Foo seems like a more natural name for the protocol/interface, since it's the foreign language equivalent of the Foo trait in Rust. Also, it's less error prone since users and UniFFI devs are likely to subclass Foo and it's not always safe to subclass the implementation type (see #2685).