Add solhint

[mariano-aguero]

This PR adds solhint for linting the solidity code.

I use the recommended config rules and disabled all styling rules (and some others, like no-inline-assembly)

This is the output right now:

contracts/Bindings.sol
  18:1  warning  Code contains empty block  no-empty-blocks

contracts/RenExAtomicSwapper.sol
   55:17  warning  Avoid to make time-based decisions in your business logic  not-rely-on-time
  109:31  warning  Avoid to make time-based decisions in your business logic  not-rely-on-time
  158:17  warning  Avoid to make time-based decisions in your business logic  not-rely-on-time

contracts/RenExBalances.sol
   75:17  warning  Open bracket must be on same line. It must be indented by other constructions by space  bracket-align
   78:22  warning  Avoid to make time-based decisions in your business logic                               not-rely-on-time
  197:55  warning  Avoid to make time-based decisions in your business logic                               not-rely-on-time

contracts/RenExSettlement.sol
  257:43  warning  Avoid to make time-based decisions in your business logic  not-rely-on-time

✖ 8 problems (0 errors, 8 warnings)

There is a private repository that I don't have access , it would be necessary to update the package lock with this repository, if I can contribute in any way with this let me know.

Best regards