Echidna tests and tooling updates

Restructured directories with Echidna test files to contain contract-specific tests
Added more Echidna tests (mostly wallet and some internals)
Updated the CI to run additional jobs
Improved Slither build script
Fixed contract flattening to include more contracts
Updated solium linter rules to ignore unnecessary warnings
Removed the ENSBuffer.sol contract as it was not used anywhere and prevented slither-flat from including it in the flattened contracts.
Regenerated contract bindings

tokencard / contracts