Collect serializations of CLOSXPs
Implement Closure (CLOSXP) class in the REXP hierarchy
Add test cases for parsing closures
Implement QAP parsing of closures (XT_CLOS)
Generate objects to serialize with GlobalEnv as enclosure

This is only of relevance when creating function objects (closures)
because with old serializations I was getting some left-over 'promise'
objects in the function's enclosing environment, which we (currently)
cannot parse.
Define the expected 'environment' attribute of test cases for closures
Implement parsing of Closures (closes #29)