add `exports:` declaration to the language. this allows users to
directly export functions marked `@external` from libraries instead of
writing `@external` wrapper functions (and consequently, for library
authors to define the external interface for their modules).

important refactors:
- remove expanded getters from module AST, change them to annotation on
  public VariablDecls.
- redo order of node visitation in ModuleAnalyzer
- refactor `InterfaceT.validate_implements()` to handle exported
  functions
- add `exposed_functions` property to `ModuleT` which reflects the
  runtime functions exposed in the selector table
- refactor IR generation to use new reachability analysis

misc:
- remove Module.add_to_body, Module.remove_from_body as they are dead
  now
- move VariableDecl validation to vyper/ast/nodes.py
- improve call-site annotations for error messages
- improve annotations for exceptions which reference a previously-
  declared node.