Fix error messages for some illegal constructions, fix absolute path includes (#251)

* Updated tests, banned type decls and toplevel letvals

* Properly ban nested contracts

* Fix including by path

* Fix error message test

* Fix prettpr attr display. Make dialyzer happy

* More tests

* Fixed type printing

* Updated docs

test/contracts/bad_arity.aes

@@ -0,0 +1,4 @@
+contract C =
+  type id('a) = 'a
+  entrypoint f() : id = 123
+  entrypoint g() : id(int, int) = 123