Support 'shadow' clause in package.

[jbouwman]

Extend the syntax of package to support shadowing of symbols:

(package
  ...
  (shadow a b c ...))

This has the same effect as a Lisp defpackage :shadow clause.

• unify package tests
  • where possible, move tests to single-file test suite in parser-test-files/package.txt
• add cursor tests
• add condition printer for cursor errors
• add source span to package AST node
• support 'good' assertions in parser test suite input files
  • if error block is blank, parser input should be accepted

Closes #1210