Expand namespacing with additional features

[b-studios]

PR #388 implements basic namespacing. In the future it would be great to have support for additional features. In particular,

• [ ] control exporting selectively using visibility modifiers. Technically this could be implemented by annotating bindings with the modifier

  case class Bindings[F[_]](
    terms: Map[String, Set[F[TermSymbol]]], // terms can be overloaded
    types: Map[String, F[TypeSymbol]],
    captures: Map[String, F[Capture]],
    namespaces: Map[String, Bindings[F]]
  )
  
  enum Visibility[T] {
    case Public(binding: T)
    case Private(binding: T)
  
    def binding: T
  }
  

• [ ] separate include (that is,include "foo/bar.effekt" as bar) and import and
• [ ] allow more fine-grained imports, i.e., import foo::bar::{ x, y }, renaming imports etc.
• [ ] add export foo for reexporting, again potentially with renaming
• [ ] add a full project mode where all (specified) files are always processed so includes are not necessary. Such a project mode could also configure other flags to the Effekt compiler, as well as declare dependencies etc.

Look at https://gist.github.com/jiribenes/8195243f56a454d3465b06f02e8bb5a2?permalink_comment_id=4872857#gistcomment-4872857 for discussions on designs.

[jiribenes]

Related to https://github.com/effekt-lang/effekt/issues/426