Fix links to 4clojure problem sets. The OG site was

discontinued by the OG maintainers, after a decade of keeping the lights
on! It has been re-hosted by @oxalorg and @lambdaisland, at
https://4clojure.oxal.org

Ref: [ANN] Discontinuing 4clojure.com
https://groups.google.com/g/clojure/c/ZWmDEzvn-Js

Author: adityaathalye

src/clojure_by_example/ex00_introduction.clj

@@ -253,5 +253,5 @@ same  ; is defined in the current ns
 ;;
 ;; - With the knowledge you have so far, you can try solving these
 ;;   problems at 4clojure.com: 1 to 13, and 16, 47, 126, 161, and 162
-;;   e.g. http://www.4clojure.com/problem/1
-;;   e.g. http://www.4clojure.com/problem/16
+;;   e.g. https://4clojure.oxal.org/#/problem/1
+;;   e.g. https://4clojure.oxal.org/#/problem/16

src/clojure_by_example/ex01_fundamentally_functional.clj

@@ -489,6 +489,6 @@ reduce
 ;;   that 4clojure requires.
 ;;
 (comment
-  (map (fn [problem-no] (str "http://www.4clojure.com/problem/"
+  (map (fn [problem-no] (str "https://4clojure.oxal.org/#/problem/"
                              problem-no))
        [14, 15, 19, 20, 48, 45]))

src/clojure_by_example/ex02_domain_as_data.clj

@@ -245,7 +245,7 @@
 ;;   that 4clojure requires.
 ;;
 (comment
-  (map (fn [problem-no] (str "http://www.4clojure.com/problem/"
+  (map (fn [problem-no] (str "https://4clojure.oxal.org/#/problem/"
                              problem-no))
        [17, 18, 57, 71
         134, 27, 26, 39]))

src/clojure_by_example/ex03_data_and_functions.clj

@@ -437,7 +437,7 @@ p/target-planets
 ;;   without code-golfing or hacks. Then translate to anonymous function form
 ;;   that 4clojure requires.
 (comment
-  (map (fn [problem-no] (str "http://www.4clojure.com/problem/"
+  (map (fn [problem-no] (str "https://4clojure.oxal.org/#/problem/"
                              problem-no))
        [37, 64, 72, 21, 24, 25,
         38, 29, 42, 31, 81, 107,

src/clojure_by_example/ex04_api_design.clj

@@ -321,7 +321,7 @@
 ;;   that 4clojure requires.
 ;;
 (comment
-  (map (fn [problem-no] (str "http://www.4clojure.com/problem/"
+  (map (fn [problem-no] (str "https://4clojure.oxal.org/#/problem/"
                              problem-no))
        [35, 36, 68, 145, 52,
         156, 22, 23, 32, 30,

src/clojure_by_example/ex05_immutability_and_fp.clj

@@ -276,6 +276,6 @@ other-pi ; what should this be?
 ;;   that 4clojure requires.
 ;;
 (comment
-  (map (fn [problem-no] (str "http://www.4clojure.com/problem/"
+  (map (fn [problem-no] (str "https://4clojure.oxal.org/#/problem/"
                              problem-no))
        [51, 77, 60, 102, 86, 115]))