clarify identifier scope, allow bare Identifiers, allow Lists and Maps

[jakemac53]

Attempt to close #2093, and #2092. Related to #2012.

• Adds Identifier, List, and Map as valid parameter types for macro constructors (and thus valid arguments for macro applications).
  • List and Map are allowed to have type arguments that are any of the supported types. This allows for List<Identifier>, etc.
• Specify the scope for identifiers in macro application arguments better (both bare and in code objects).
• Some other unrelated cleanup (can remove if desired).
  • Fixed up some old links
  • Removed the section on Fragment (you can just use Code for this now).