Allow pairlist CDRs that are not lists

E.g., a closure is saved as a pairlist, but the final element is the
body, which is a single Language instance.
Fix parsing of serialized closures (close #34)

A closure constructed by `parse`+`eval` of source text includes
a 'srcref' attribute that links the code to the original source. We
weren't properly accounting for the presence of these attributes
during the parsing of serialized closures, but now we do.
Add notes on the serialization of closures