Add tests for correctness of YAML output

[jasom]

Since the parser is fairly well tested, we should just be able to mostly just test that round-tripping lisp data comes back correctly.

Some extra work will be needed to make sure that lists and a-lists are correctly handled; and there is a corner-case where

[[a,b],[c,d]] will be ambigous with {a:[b],c:[d]} when alists are in use. Perhaps we should have an option in the emitter to prevent detection of alists?