Remove duplicated lamda/function pointer rules

[lucic71]

stable_sort rules had 2 version of rules: one that takes a lambda and one that takes a function pointer. This PR removes the duplication by adding a new trait in libcc2rs: Callable.

Callable is implemented by Fn (closures, safe function items, safe function pointers) and unsafe fn pointers. Unsafe functoin items reach the Callable trait to an explicit cast to unsafe fn pointers because they don't implement Fn.