docs: split monolithic core-library.markdown into per-class docs under doc/site/core/

doc/site/core/fn.markdown

@@ -0,0 +1,23 @@
+^title Fn Class
+^category core
+
+A first class function—an object that wraps an executable chunk of code.
+[Here](../functions.html) is a friendly introduction.
+
+### new **Fn**(function)
+
+Creates a new function from... `function`. Of course, `function` is already be
+a function, so this really just returns the argument. It exists mainly to let
+you create a "bare" function when you don't want to immediately pass it as a
+[block argument](../functions.html#block-arguments) to some other method.
+
+    :::dart
+    var fn = new Fn {
+      IO.print("The body")
+    }
+
+It is a runtime error if `block` is not a function.
+
+### **call**(args...)
+
+**TODO**