I think C-M-u is doing what I would expect, although it is currently moving point to "{" instead of the beginning of the function definition (in .

But I like the idea of using C-M-u/C-M-d to move outwards and inwards. And maybe we can use C-M-a/C-M-e for horizontal tree movements across sibling functions, or upwards when the current level does not have sibling functions.