Allow runtime values for argument and main annotations

[lihaoyi]

Currently this is not possible because we use ClassfileAnnotation, which only allows constants. StaticAnnotation would in theory let us use expressions, but that doesn't work for annotating arguments while using named arguments in the annotation due to https://users.scala-lang.org/t/how-to-use-named-arguments-in-scala-user-defined-annotations/4163.

A workaround for this would be to explode the singular @arg annotation:

class arg(val name: String = null,
          val short: Char = 0,
          val doc: String = null,
          val noDefaultName: Boolean = false,
          val positional: Boolean = false) extends ClassfileAnnotation

Into a collection of single-value annotations:

class name(value: String) extends StaticAnnotation
class short(value: Char) extends StaticAnnotation
class doc(value: String) extends StaticAnnotation
class noDefaultName extends StaticAnnotation
class positional extends StaticAnnotation

Using this style, a user would be able to specify things about an argument without needing to use named arguments, allowing us to workaround both problems and provide non-constant runtime values as part of the annotation metadata