Produce stub and missing function reports

[markrtuttle]

• Create a "proof assumptions" section in report that lists all expected stubs and expected undefined functions. The definition of expected comes from the cbmc-viewer.json proof configuration file.
• Report unexpected stubs and unexpected undefined functions as errors and alarm on them.
• Summarize this information across all proofs on the litani report with histograms.

[markrtuttle]

Could also include expected coverage.

[markrtuttle]

A preliminary implementation now appears in the summary directory. This needs to be cleaned up and merged into viewer iteself.