Calling by method name on a union of distinct types with distinct capabilities.

I was doing some brainstorming about a common interface for "byte writers" that would cover both `class ref` and `actor tag` implementations of methods for writing out bytes. I quickly figured out that I couldn't have the same interface cover both cases, but I thought a type union of two interfaces could work well, so I created `OutClass` and `OutActor` interfaces, and called the union of them `Out`:

```pony
type Out is (OutActor tag | OutClass ref)

interface tag OutActor
be print(string : String)

interface ref OutClass
fun ref print(string : String)

class StringWriter is OutClass
var _string: String iso = recover String end

fun ref finish(): String iso^ => _string = recover String end
fun ref print(string : String) =>
_string.append(string)

actor Main
new create(env: Env) =>
let out_1: Out = env.out
out_1.print("Hello, world!")

let out_2: Out = StringWriter
out_2.print("Hello, world!")
try env.out.print((out_2 as StringWriter).finish()) end
```
Doing so gives this error output:
```
Error:
main.pony:19:16: receiver type is not a subtype of target type
out_1.print("Hello, world!")
^
Info:
main.pony:19:5: receiver type: (OutActor tag | OutClass ref)
out_1.print("Hello, world!")
^
main.pony:7:3: target type: (OutActor ref | OutClass ref)
fun ref print(string : String)
^
main.pony:1:14: OutActor tag is not a subtype of OutActor ref: tag is not a subcap of ref
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: OutActor tag is not a subtype of OutClass ref: tag is not a subcap of ref
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: OutActor tag is not a subtype of any element of (OutActor ref | OutClass ref)
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: not every element of (OutActor tag | OutClass ref) is a subtype of (OutActor ref | OutClass ref)
type Out is (OutActor tag | OutClass ref)
^
Error:
main.pony:22:16: receiver type is not a subtype of target type
out_2.print("Hello, world!")
^
Info:
main.pony:22:5: receiver type: (OutActor tag | OutClass ref)
out_2.print("Hello, world!")
^
main.pony:7:3: target type: (OutActor ref | OutClass ref)
fun ref print(string : String)
^
main.pony:1:14: OutActor tag is not a subtype of OutActor ref: tag is not a subcap of ref
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: OutActor tag is not a subtype of OutClass ref: tag is not a subcap of ref
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: OutActor tag is not a subtype of any element of (OutActor ref | OutClass ref)
type Out is (OutActor tag | OutClass ref)
^
main.pony:1:14: not every element of (OutActor tag | OutClass ref) is a subtype of (OutActor ref | OutClass ref)
type Out is (OutActor tag | OutClass ref)
^
```

Note that if I change the code to use the specific interface rather than the type union, everything works okay:
```pony
actor Main
new create(env: Env) =>
let out_1: OutActor = env.out
out_1.print("Hello, world!")

let out_2: OutClass = StringWriter
out_2.print("Hello, world!")
try env.out.print((out_2 as StringWriter).finish()) end
```
```
Hello, world!
Hello, world!
```

Start with the minimal Pony example in the issue and inspect how the compiler resolves method calls on unions with differing capabilities. Define the expected behavior for OutActor tag | OutClass ref and capture it in a regression test; the issue does not name source files or existing tests.

Written by the indexing model from the issue text.