some miscellaneous work

changelog

• add an inspect command to komodo through main.rs
• remove the useless Shard.mul implementation and tests
• because i is a u128, use i.to_le_bytes() instead of [i as u8] in calls to E::ScalarField::from_le_bytes_mod_order
• add to tests/cli.nu the cases tha should fail
• merge together the tests in lib.rs, e.g. verify_2, verify_4 and verify_6 become a simpler verification
• add some documentation and NOTEs
• implement Display for Block to dump it to stdout
• print more detailed test cases when a test fails