isModuleEnabled view function

[gitpusha]

Hi,

It would be cool to have a view state-reader function like isOwner just for whitelisted modules on the ModuleManager.

Something might look something like this:

    function isWhitelistedModule(address module)
        public
        view
        returns (bool)
    {
        return module != SENTINEL_MODULES && modules[module] != address(0);
    }

This could be useful, if you want to perform some sanity checks on-chain. Say for example, you only want GnosisSafeProxies to call your module's mint function, if those callers also have the contract already whitelisted as a module. Normally this should be the case already, but we cannot be 100% certain of that on-chain, unless we have a way to check the whitelist.