lib: introduce foreach = flip map

The main purpose is to bring attention to `flip map`, which improves
code readablity. It is useful when ad-hoc anonymous function
grows two or more lines in `map` application:

```
      map (lcfg:
        let port = lcfg.port;
            portStr = if port != defaultPort then ":${toString port}" else "";
            scheme = if cfg.enableSSL then "https" else "http";
        in "${scheme}://cfg.hostName${portStr}"
      ) (getListen cfg);
```
Compare this to `foreach`-style:
```
      foreach (getListen cfg) (lcfg:
        let port = lcfg.port;
            portStr = if port != defaultPort then ":${toString port}" else "";
            scheme = if cfg.enableSSL then "https" else "http";
        in "${scheme}://cfg.hostName${portStr}"
      );
```
This is similar to Haskell's `for` (http://hackage.haskell.org/package/base-4.12.0.0/docs/Data-Traversable.html#v:for)

lib/default.nix

@@ -71,7 +71,7 @@ let
       zipAttrsWithNames zipAttrsWith zipAttrs recursiveUpdateUntil
       recursiveUpdate matchAttrs overrideExisting getOutput getBin
       getLib getDev chooseDevOutputs zipWithNames zip;
-    inherit (lists) singleton foldr fold foldl foldl' imap0 imap1
+    inherit (lists) singleton foreach foldr fold foldl foldl' imap0 imap1
       concatMap flatten remove findSingle findFirst any all count
       optional optionals toList range partition zipListsWith zipLists
       reverseList listDfs toposort sort naturalSort compareLists take