Make nothing :: Maybe a, remove extra parameter

`nothing` is only used in one place, and as a function it was called
incorrectly before: `nothing()` instead of `nothing(undefined)`.

This commit fixes that by removing the extra layer of function wrapping
entirely, and making `nothing` a constant of type `Maybe a`. This is to
avoid the more verbose alternative - adding the `undefined` argument.

Author: eush77

appendix_a.md

@@ -141,8 +141,8 @@ const maybe = curry((v, f, m) => {
 ## nothing
 
 ```js
-// nothing :: () -> Maybe a
-const nothing = () => Maybe.of(null);
+// nothing :: Maybe a
+const nothing = Maybe.of(null);
 ```
 
 

exercises/ch11/solution_a.js

@@ -1 +1 @@
-const eitherToMaybe = either(_ => nothing(), Maybe.of);
+const eitherToMaybe = either(always(nothing), Maybe.of);

exercises/support.js

@@ -579,7 +579,7 @@ const maybe = curry(function maybe(v, f, m) {
   return f(m.$value);
 });
 
-const nothing = function nothing() { return Maybe.of(null); };
+const nothing = Maybe.of(null);
 
 const reject = function reject(x) { return Task.rejected(x); };