Treat SAM classes's `(using ...)` argument lists as contextual

[makkarpov]

Currently, constructs like Foo ?=> (Bar, Baz) => Qux causes two function objects be allocated on heap. This may be especially costly on repeated invocations, as there is no way to cache second function. Usually, performance-sensitive spots are highly specialized, so someone might want to replace such declaration with a specialized trait:

trait FooFunction {
  def apply(using Foo)(a: Bar, b: Baz): Qux
}

However, currently it doesn't work as expected:

object Meow extends App {
  trait FooFunc {
    def apply(using Foo)(b: Bar): Unit
  }

  val f: FooFunc = b => println(s"bar=$b, foo=${summon[Foo]}")
  // error: expected Bar => Unit, got Unit
}

Feature request is to unify behavior of such SAM lambdas with currently present behavior of context functions.