Add mean function to boot.janet

Update changelog.

CHANGELOG.md

@@ -2,6 +2,8 @@
 All notable changes to this project will be documented in this file.
 
 ## Unreleased
+- Fix an issue that prevent some valid programs from compiling.
+- Add `mean` to core.
 - Allow (length x) on typed arrays an other abstract types that implement
   the :length method.
 

src/boot/boot.janet

@@ -445,6 +445,11 @@
   (each x xs (+= accum x))
   accum)
 
+(defn mean
+  "Returns the mean of xs. If empty, returns NaN."
+  [xs]
+  (/ (sum xs) (length xs)))
+
 (defn product
   "Returns the product of xs. If xs is empty, returns 1."
   [xs]