Compile Lua's ... form.

* module/language/lua/compile-tree-il.scm (compile): Add clause for
  ast-variable-arguments.
* module/language/lua/parser.scm (define-ast, make-parser): Add
  vararg-gensym field to functions, gensym field to variable-arguments.
  Propagate *vararg-gensym* from functions to variable-arguments.
* test-suite/tests/lua-eval-2.test ("lua-eval"): Check for #nil