Check that generators are created with valid source and target types.

* new/builtin.jam
    (lib-generator): Don't pass any source types to base's ctor.

* new/generators.jam
    (generator): Check types.
    (composing-generators): Allow empty list of source types.

* new/type.jam
    (validate): New rule


[SVN r17168]

tools/build

@@ -1 +1 @@
-Subproject commit bf562a5128556c050a21742cea9103327b5d2cd6
+Subproject commit 3a6503758595374e5e405fb045accbdf84bd6ec4