either: add S.lefts and S.rights

[davidchambers]

This pull request adds Haskell's lefts and rights:

lefts  :: [Either a b] -> [a]
rights :: [Either a b] -> [b]

I recently reviewed some code @Bpless wrote which could be nicely expressed in terms of rights.

These functions are similar to S.catMaybes, which should arguably be renamed S.justs for consistency.

[Bpless]

In both of these cases, we could alternatively filter -> map to achieve the same ends.

[davidchambers]

We certainly could!

   def('lefts',
       {},
       [$.Array($Either(a, b)), $.Array(a)],
-      R.chain(function(either) {
-        return either.isLeft ? [either.value] : [];
-      }));
+      compose(R.pluck('value'), R.filter(isLeft)));

   //# rights :: [Either a b] -> [b]
   //.

I slightly prefer the chain version because it avoids an intermediate collection. :)

[svozza]

I prefer chain too.

[svozza]

These a nice functions, I could see them being quite useful.