Overhaul assertions
- Close keybase/keybase-issues#971
- Close keybase/keybase-issues#972
- Now you can make conjunctions via '&&' and disjunctions via '||'
- Grammar now is "twitter://maxtaco" and not "twitter:maxtaco"
- Parentheses also supported
- Only one "--assert" arg needed (since conjunctions as above).
- No warnings on unmatched proofs