Fuzz fixes

[sanket1729]

1. Removes redundant tests from fuzz repo: roundtrip_policy.rs had the same code which was split in roundtrip_semantic and rondtrip_concrete
2. Removes other replicated code for crash replication from each fuzz file. Added a README with instructions for crash replication on Travis.
3. Fixes fuzz_targets/roundtrip_policy.rs fails with 6f7228706b282929 (~ "or(pk())") #92 by adding a regex to remove '1@' from the strings. Basically, it looks for all occurrences of \D1@ a non-digit followed by 1@ and replaces them with "". Other solutions to fixing fuzz_targets/roundtrip_policy.rs fails with 6f7228706b282929 (~ "or(pk())") #92 might have involved leaking some internal tree parsing code outside of repo for testing purposes.
4. Suppressed the dyn warnings. I don't know what is your opinion about these.
5. Fixed bench tests and added those to travis.

Can rebase and squash later if desired.

[apoelstra]

ACK.

Heads up that eventually we do want to support n-ary and and or ... but this is a fairly low-priority thing for me.