Reorganize site to make room for documenting the other built-in modules.

doc/site/modules/core/fn.markdown

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+^title Fn Class
+
+A first class function—an object that wraps an executable chunk of code.
+[Here][functions] is a friendly introduction.
+
+[functions]: ../../functions.html
+
+### Fn.**new**(function)
+
+Creates a new function from... `function`. Of course, `function` is already 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.
+
+    :::wren
+    var fn = Fn.new {
+      System.print("The body")
+    }
+
+It is a runtime error if `function` is not a function.
+
+## Methods
+
+### **arity**
+
+The number of arguments the function requires.
+
+    :::wren
+    System.print(Fn.new {}.arity)             //> 0
+    System.print(Fn.new {|a, b, c| a }.arity) //> 3
+
+### **call**(args...)
+
+Invokes the function with the given arguments.
+
+    :::wren
+    var fn = Fn.new { |arg|
+      System.print(arg)     //> Hello world
+    }
+
+    fn.call("Hello world")
+
+It is a runtime error if the number of arguments given is less than the arity
+of the function. If more arguments are given than the function's arity they are
+ignored.