Added new ceylon classpath tool for #1640
Model loader: try to substitute declarations we can't load (with reflection model loader) with UnknownType

To prevent entire packages from being unloadable if a single class has link issues. In theory
this may mean that only when accessing the missing class will we get an excepion. In theory.
Merged all language tests in a single runner

Much faster compilation
NumberImpl: added missing new members
Add test for both ceylon/ceylon.language#370 and #1640

They are done as part of the LanguageSuite so we don't have to compile
the test suite more than once, even though we end up running it more than
once. These test the classpath tool and use it to run the tests in a new JVM
without JBoss modules or custom class loaders.