Minor HLint-suggested changes

Some minor changes suggested by HLint:

* Use `all` instead of `and . map`;
* Use `elem` and `notElem` instead of chains of `x == 'a' || x == 'b' || x == 'c'`, etc.;
* Remove redundant lambdas;
* Remove parens around single items;
* Use `/=` instead of `not (... == ...)`;
* Use `fromMaybe x` instead of `maybe x id`.