Generate cl.Null for "null" class
All invocation code is now in this class. Fix union type TODO
runtime check for dynamic specifier expr for #170
deal better with unknown types (reify to Anything)
add runtime check for method invocations #170
Add dynamic/typed runtime check for sequenced enums, tuples and
attribute declarations #170
tests for #170
more tests for #170
runtime dynamic check when reassigning values to variable attribs for #170