drop Function, rename FunctionBody -> Function

[stepancheg]

If someone needs Function to be refcounted, Function can be explicitly wrapped in sharedpointer, like this:

var functionPtr = new(Function((x: Int) -> x * 2));
var thirtyFour = functionPtr^(17);

Note that sharedpointers are not thread-safe (thus Function weren't thread-safe until this patch), so simple function copying could cause memory problems.

[jckarter]

Sounds good. Separating the ownership policy from Function also allows you to use it with unique pointers, which is useful.