Add rule for detecting redundant fallback function

[fvictorio]

Adding a fallback function that just reverts:

function () external payable {
  revert();
}

is the same as not adding it (and maybe the bytecode is unnecessarily larger? I don't know if the compiler is smart enough to detect this).

We could have a rule for detecting this. The rule should check if the fallback function has only one statement and if that statement is a call to revert.

The only potential issue I see is if you add a revert function to your contract and you want that function to be called in the fallback function. This would trigger a false positive. We could check if there's a function revert in the contract, but the function could be inherited from another contract. So I wouldn't bother: if someone is doing this, then they can just disable the rule.

Possible names:

• no-redundant-fallback
• no-unnecessary-fallback
• dude-or-gal-check-the-docs-you-can-drop-that-code
• Other?

cc @patitonar @fernandomg @pablofullana