Enable structured output of verification results from CBMC

[NlightNFotis]

As it stands, CBMC reports results in a variety of formats, but all of them are more or less a textual stream, which makes it less amenable to programmatic manipulation than some structured output format.

This particularly affects the consumers of the APIs of CBMC.

[martin-cs]

In principle the two machine readable formats --xml and --json should be sufficiently structured. --graphml is worth a look as well.