compiler support for letrec*

compiler support for letrec*

* test-suite/tests/tree-il.test ("letrec"): Add some tests.
* module/language/tree-il/compile-glil.scm (flatten): Add support for
  compiling letrec* in its unoptimized form.

* module/language/tree-il/fix-letrec.scm (simple-expression?):
  Parameterize, so that letrec* will not treat `(car x)' as primitive
  (because it could raise an exception).
  (partition-vars): Lump unreferenced vars in with complex vars, when
  compiling letrec*.
  (fix-letrec!): No need to evaluate inits within a let for letrec*.