Define a Display (like) trait for the Verifier errors

[nick-mobilecoin]

In #2 we settled on an error tree that contains all the errors about verifying data.
We want some way to display this in an ergonomic way so that humans can more easily fix verification issues.
One thought is out put like

Report Error:
  and: 
    Attributes failed (expected, actual)
    and:
      SVN (passed)
      product ID (passed)

The above is a very rough draft, the idea being that the error tree can be visually represented and humans can quickly find the failures.
it may be that only failures are shown.

• Display traits on SGX structs sgx#319
• Add display capability to Verifier output #31
• Add VerificationTreeDisplay #48 This includes
  • Not display
  • Mrsigner display
• Need accessor methods for Attributes, in Add VerificationTreeDisplay #48 going to the wrapped C struct and accessing the fields directly This is now handled by the Display trait on Attributes
• Move common types to lib.rs #86
• Convert from CtOption to VerificationOutput #87