Use Candiff to compare logs

[matthewhammer]

Addresses #7

• Save and emit the call request-response log to compare across distinct CI runs
• Download candiff release binary in CI script (avoid rebuilding from source, which is slow)
• Compare logs across runs

[matthewhammer]

This PR is passing CI, but candiff is barfing on what seems to be a minor mismatch between didc and candiff syntax expectations.

[matthewhammer]

Merging this now.
Loose end for another PR:

• As discussed with @chenyan-dfinity, there is a mismatch between IDLArgs and IDLValue between the two tools. Seems like an easy fix with an appropriate flag in the candiff tool, which currently expects the latter but should understand the former.