Added substr function.

This closes #86.

stdlib/stdlib/string-substr.lisp

@@ -0,0 +1,19 @@
+;;; string-substr.lisp - Fetch substrings from a string
+
+;; String handling is good to have, and here we implement substr in the
+;; naive way:
+;;
+;; Split the string into a list, and take parts of it using "take" and
+;; "drop".
+;;
+
+
+(set! substr (fn* (str start &len)
+                  "Return a substring of the given string, by starting index.
+
+The length of the substring is optional."
+                  (if (> start (strlen str))  ; out of bounds?
+                      ""
+                    (if (nil? len)  ; start at the given offset
+                        (join (drop start (explode str)))
+                      (join (take (car len) (drop start (explode str))))))))