Add non-nestable OR patterns without bindings

Support non-nestable OR patterns without bindings such as:
```scala
str match {
"YES" or "NO" => ...
_ => ...
}

car match {
case Car("Mercedes-Benz", _) or Car("smart", _) => ...
_ =>
}

people match {
case Cons("Joe", Nil()) or Cons("Jolene", Nil()) => ...
...
}
```

only on the top-level (no nesting -- much like `and`) and for patterns **without** bindings (those should be rejected).
This way, we get the 80% of use cases with 20% of the work of implementing actual nested OR patterns with bindings.

Then this change should be somewhat simple to add, just duplicating the branches.

No files or tests are named. Start by locating the compiler's pattern-matching implementation and the existing handling for top-level `and` patterns. Done means supporting the shown non-nestable OR patterns without bindings while rejecting OR patterns with bindings.

Written by the indexing model from the issue text.