Linting integration

This commit brings in an HLint configuration file
and several recommended modifications such as:

* End-of-line extra spaces removal;
* Import lines ordering;
* Redundant $ removal;
* Generalisation of ++ and map to <> and fmap;
* Preferring `pure` over `return`;
* Removing extraenous extensions.

And finally, a `stylish-haskell` helper script
that detects if code files are dirty. Can be useful for CI,
although manually calling it can be nice if you would rather
first implement then beautify.

.hlint.yaml

@@ -0,0 +1,8 @@
+- group: {name: generalise, enabled: true}
+
+- warn: { name: Use DerivingStrategies }
+
+- error: { lhs: return, rhs: pure }
+
+- ignore: {name: "Avoid lambda using `infix`"}
+