Parser: Fix long identifiers

Long identifiers are not supposed to *require* dots:

  > open System
  line 1:5 mismatched input 'System' expecting LongIdent
  Exception in thread "main" java.lang.NullPointerException
          at fjord.ast.ScriptFragment.accept(ScriptFragment.java:15)
          at fjord.Main.eval(Main.java:33)
          at fjord.Main.main(Main.java:22)

Fix that up.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
AST: Fix NPE in ScriptFragment#accept

Signed-off-by: Pekka Enberg <penberg@kernel.org>